@slartabartfast:

I agree that at least some of the limitations seem to have been from the sound card.  I don't want to spend gazillions on a sound card (I just use the computer for creating sound files -- if I want to listen to music, I use what in the olden days was called a hi-fi -- and I don't plan to use it to record live music), which is why I was wondering if something like a Xonar Essence STX would be sufficient for my needs.

Not that I think you will not find a new computer and sound card a valuable investment, but I am pretty sure that your soundblaster would be sufficient for your needs. If you do not understand how to play multiple tracks simultaneously in Sonar 3, then you will not be able to do it in Sonar X1 either. The sound card you are using is completely irrelevant. This is not a hardware problem, and it will not be solved by buying newer or better hardware.
 
What are you using to turn your midi file into audio data? Are you using the soundblaster synthesizer or a soft synth, sampler or rompler? Even with the lame Soundblaster mixer and synth, you should be able to record multiple audio tracks generated in the Soundblaster and play them back all at once from Sonar 3.
 
It has become pretty obvious that you are trying to send the output of a track in Sonar 3 directly to your sound card. If your sound card driver is set up to only accept two connections from Sonar, and you send one track to stero right and one track to stereo left and then try to send a third track directly to the sound card there is no place to send it to--you used them up with the first two tracks. The solution with Sonar and with almost every other audio software available is to NEVER SEND OUTPUT FROM A TRACK DIRECTLY TO THE SOUND CARD.
 
First Principles: You can have dozens of tracks on your computer but when you listen to the combined tracks in stereo they all have to come out of two speakers. Somewhere they have to be mixed together. Where do they get mixed? In your case the answer is in the computer and specifically in Sonar 3. When do they get mixed? Before the stereo signal is sent to the sound card. Believe me it is possible to play many tracks simultaneously through your sondblaster if you have your sonar tracks set up correctly.
 
A midi track has to send its output to a synthesizer of some sort. The synthesizer has to send the audio data it creates in response to the midi file control to an audio track. The audio track has to send its audio data to a sound card eventually, but it does not have to send its audio data directly to the sound card.
 
Instead of setting the output of an audio track to your sound card, send the track audio output to a BUS. A bus is an electrical term for a place where a lot of circuits are connected together, in an analogue mixer it is exaclty that. In audio software, a bus is a program module that combines inputs from several sources (tracks) "adds" them together and passes the mixed signal to another location. A bus will accept lots of track ouputs. For this purpose you can have one bus for each of the sound cards audio connections. So set the output of any track you want to hear in the mix to the bus, then set the output of the bus to the sound card.
 
http://www.soundonsound.com/sos/dec03/articles/sonarnotes.htm
 
Signal flow in Sonar is a lot simpler and logical than music notation, but you do need to spend some time understanding how it works.

@slartibartfast:

> If you do not understand how to play multiple tracks simultaneously in Sonar 3,

As Marge Gunderson says, you have no call to get snippy with me.  Of course I know how to *play* multiple tracks simultaneously in Sonar 3 -- the main reason for my wanting an upgraded computer is that if I play more than a couple at a time, the CPU crashes, and no amount of fiddling with latencies and buffers or stopping other programs (e.g. antivirus) that might prevent SONAR from using its fair share of the CPU seems to help.  (IOW, despite all appearances to the contrary, I am in fact not a moron -- just someone who is about a decade behind the times.)

> What are you using to turn your midi file into audio data? Are you using the
> soundblaster synthesizer or a soft synth, sampler or rompler?

I generally use Garritan Personal Orchestra for solo instruments and Edirol for section strings and percussion.

>  In your case the answer is in the computer and specifically in Sonar 3. When do they
> get mixed? Before the stereo signal is sent to the sound card. Believe me it is possible to
> play many tracks simultaneously through your sondblaster if you have your sonar tracks
> set up correctly.

Yes, and I manage to do that successfully.  But at least the way my old computer is set up, I have to assign each one of the Sonar tracks to a MIDI I/O channel of the sound card.  I suppose that could have been a mistake on the part of the people who sold me the computer, but that's the way it is -- the only option for MIDI device that I have (at least the only one that actually leads to a sound coming out of the speakers) is the Soundblaster. 

> Signal flow in Sonar is a lot simpler and logical than music notation, but you do need to
> spend some time understanding how it works.

Which is kind of the whole point of my initial query:  since it's been nearly a decade since I bought a computer, I was under the (possibly mistaken) impression that improvements in Sonar and possibly in hardware (e.g. the sound card) and operating system (Windows 7.0 vs. XP) may have made it possible to create a signal flow that is simpler and more logical than it was at the time that I got my previous computer.  Before I invest the money in a new computer (and in an updated version of Sonar), I am trying to figure out what system I can design that will best meet my needs.

If you tell me that the treatment of the MIDI signals no longer needs the involvement of the sound card, then I can accept that.  If you tell me that it never did and that I was misled by an incorrect setup of my computer into thinking that all MIDI I/O had to go via the sound card, I can accept that as well (although I remember being told at the time that sound cards that lacked MIDI I/O were less user-friendly for music editing than Soundblaster cards were). 

If all I need to know is that for editing a MIDI file produced by a notation program like Sibelius or Finale, the sound card doesn't matter, and that I will find that any sound card equally compatible with and equally easy to use with Sonar, then that's all that you have to tell me without putting on airs about your superior knowledge.  The issue then becomes which sound card is most compatible with Sibelius.  The people at Sibelius claim that the ASUS ASIO drivers don't work, and that an ASUS sound card will cause Sibelius to crash.  I have been unable to get independent confirmation for that claim -- it might be true, but on the other hand, M-Audio and Sibelius are both sold by Avid, so the people at Sibelius certainly have an interest in convincing me not to buy an ASUS sound card.

P.S.  If you still think that I can't get multi-channel playback from Sonar 3.0, listen to the mp3 at http://www.scoreexchange.com/scores/14962.html, which was created from a wav file that was created using Sonar 3.0 (despite the program's crashing every 5 seconds whenever I would try to get the full orchestra to play).

I apologize for underestimating your abilities balock63. Sometimes in trying to be helpful, without knowing who is at the other end, I make incorrect assumptions. You are not the first to note that the tone of my advice sometimes seems like putting on airs--some would say self-aggrandizing supercilious condescension is more apt. I was not trying to be deliberately insulting. 
 
I was, and remain, confused about the original idea that a sound card with more independant channels would help with your problem. A better sound card might be useful in a lot of ways, but the sound card is probably not at the root of your performance problems.
 
If you are having overloads/dropouts while attempting to play a lot of tracks through your main out bus, then a better processor/motherboard would help. Even more important than a processor upgrade for someone who is using a lot of memory-resident samples, would be more addressable memory. That means you need to run a 64-bit Windows system. Windows 7 -64 would be the most reliable system for that. You can get access to 16 GB with the Home Premium version. That should be more than enough. Although the Professional version will address 192 GB of memory, there is no motherboard made that will accomodate that much.
 
 

Thanks for your advice. I am hoping that a 64-bit Windows system with an i7-3930K processor and 32 GB of RAM will solve at least some of my problems -- I've been warned that even that may not be enough to have an "orchestra" running without crashing the system ("buy a bunch o' computers and run them in parallel, with one dedicated to strings, one to brass, one to woodwinds, and one to percussion" was the advice I was given). As I said, the source of my confusion appears to have been that in my old system, for whatever reason, the only MIDI I/O that I had was via the sound card. If in the last decade, that's become irrelevant, than all I can say is thank you to whoever is responsible for the improvements in technology. If I understand you correctly, since I don't actually do any live recording (although I have thought of using something like SoundForge to digitize my LPs -- you can see what a Luddite I am), the actual sound card isn't all that important as far as SONAR is concerned. The only question remaining then is how much difference the motherboard makes. Since most sound cards in the price range that I can afford still have PCI interfaces, that means that instead of a P9X79 "deluxe," I would have to use the plain old P9X79. Naturally, the P9X79 only has two 6-Gb SATA connections, which means that if you want an SSD and two HDDs, one of the HDDs is going to be connected to the slower port (not, I suppose, that it would make a measurable difference). Or are there decent, reasonably priced PCIe sound cards? (The ASUS Xonar is out of the question because of the ASIO problems I mentioned earlier.)

I am hoping that a 64-bit Windows system with an i7-3930K processor and 32 GB of RAM will solve at least some of my problems -- I've been warned that even that may not be enough to have an "orchestra" running without crashing the system

This is exactly what I'm running (see sig)

I recently finished building my orchestral template based around EWQLSO Platinum Plus.
The template consists of about 7/8 instances of the Play Engine and I have loaded up more articulations than I probably need, and so far ir barely tickles my cpu, RAM is reporting 7.7Gb

I have yet to actually compose anything, though I don't foresee any problems.

A couple of years ago, it would have been necessary for this scenario to require multiple computers running different parts of the libraries, but not any more.