My 9 year old computer is finally giving up the ghost, and I'm looking to get a new one that's optimized for use with the latest version of Sonar.  Since I'm not a hard-core gamer, I assume that any system that will run Sonar will run anything else that I would do (which will almost certainly be less demanding of CPU).  So here are my four questions:
 
(1) I'm planning to start with an i7 processor.  Should I expect any significant difference in performance depending on which flavor of i7 I opt for, in particular, will 6 cores (if such a thing exists anywhere outside of Intel's web site) give enough of an improvement in performance over 4 to be worth the additional cost?
 
(2) What motherboard do people recommend (assuming that it actually makes a difference)?
 
(3) Is there any sound card that people like, both for overall quality and for ease of use with Sonar?  In my ancient computer, I have a Creative SoundBlaster Audigy 2 and in the version of Sonar that I'd been running (with an embarassingly small version number), I could never figure out how to get it to use all of the channels of the sound card.
 
(4) Are there any other hardware issues that I should be aware of, and if so, what are they?
 
Thanks for any advice you can give. . .

I'm using an i7 920 cpu and it works just awesome for me. Downside of this cpu is the memory, had trouble with 1600mhz and Intel tech told me that the i7 don't support other than 1033mhz, I did change it and all is good now. I think some guys are using faster memory. As for a sound card I'm using the Delta 1010lt and also works very good.

Thanks for the reply.  How many tracks does the 1010lt support?  I was thinking of getting an ASUS Xonar card, which can process up to 128 independent sounds.  Or does this no longer matter in Sonar?  (I had been using an ancient version, in which the number of independent tracks in your sound file could not, AFAIK, exceed the maximum number of independent sounds that the sound card could support.)

I was thinking of getting an ASUS Xonar card, which can process up to 128 independent sounds.

 
If the machine is specifically to be a DAW, get a dedicated pro audio interface.  The Asus card is not what you want.
Choose a unit that's a known rock-solid unit (RME, MOTU, etc)... and you'll never give it a second thought.
 
Modern i7 CPUs have enough DSP power to run multiple soft-synths.
 
 

Go to this site to choose your chip http://cpubenchmark.net/high_end_cpus.html
 
They rate the biggest bang for your buck.  When I built mine it was the 2600-K.  K being overclockable style chip.
 
Choose your motherboard wisely due to the fact the PCI sound cards are some of the best (fastest).
However, my Lynx card is PCI not the new PCIe.  So I had to choose a legacy Mother Board that had that slot. 
 
Also, I don't recommend building a mini tower anymore.  My build was a mini tower because it fits in my insulated cabinet by my desk to keep the fans quite (its a ported cabinet for fresh air).  To  hard to find parts like chip cooler fans for overclocking etc....
 
Take a close look at using SSDs.  It's the wave of the future.  Sonar's automatic backup and windows happens so fast- I don't even see it happening at all.
 
Also,  think about a good video card for multi monitors.  Its all in windows 7 now and you don't even need any other software to run them, just the video card driver.
 
Just make sure you start on paper first.  everything has to say that it is for whatever part you are matching.  Chip > Motherboard has chips socket> case fits Motherboard> etc...etc....
 
And like you probably have learned over the years get the most you can afford at the time because the software will keep getting more and more complicated (if history stays on course).
 
I saved pretty good $ building my own but it took a long long time.  When you count  research research, research and ordering with a return,  etc...etc...   ; was it all worth it- yes because now I feel like I know a lot more about computers.
 
P.S.  Oh and be very careful.  For example; did you know that Ram sticks all have to be a matched set?  I didn't. 
 
Hope this helps

Thanks to Jim and Wave. Actually, my new computer will probably end up not being dedicated to SONAR, which is why I'm planning to get a dual-boot system: the OS on one drive will be for SONAR and will be used with the computer disconnected from the Internet (so that anti-virus etc. won't be there to eat CPU), and the OS on another drive will be for general use. I actually have a sound card-related question that's admittedly on the verge of being moronic. With my current system, I compose using Sibelius and then export a MIDI file into an ancient version of SONAR (that's why I want a sound card with a lot of independent channels -- one for each staff). Many "modern" sound cards generally don't have MIDI inputs. Does that mean that that I need to get a MIDI card, or a MIDI file imported into SONAR playback even without a separate MIDI card? And if I don't want an ASUS Xonar sound card, what sound card do I want? I don't normally use my computer for general music listening and movie watching activities. But I do use it to "play" orchestral scores created in Sibelius. Maybe I'm misunderstanding the information on the web sites, but it seems that even the high-end MOTU and RME cards have fewer I/O channels than my old SoundBlaster Audigy did. Thanks again.

Not to sure about using two drives for Anti-virus; due to the fact that the virus could possible move into the other drive (but it sounds like a cheap solution).  You may be ok here,  it just occured to me you could still scan both drives (think about down time for scan though).
What I did was use a older computer for the internet and have VGA and DVI going to one monitor and just switch back and forth (using the monitor's input menu button) between the two computers.   
When your talking about Midi inputs going into the sound card (cable wise) It is just for an instrument/hardware (my Midi keyboard controller has USB into Computer not sound card). 
 
You  don't need any Midi built into the card to run midi files.  Thats a computer language that is within Sonar nothing to do with the sound Card.  Cards do have inputs for Midi and some even have Midi software.  But you don't have to have it with Sonar - As long as your instrument is not Midi out (cable).
 
As far as making music Midi files and importing them into Sonar each with its own channel out to your "Sound Cards Mains" you can do it that way but my lynx card has 8 right and left stereo channels.  I would be limited to that.
 
However, a option may be to Solo each Midi track using Sonar's "Exclusive Solo Mode" with "Solo Override" on the tracks you want to always hear.  When you click on a solo button it shuts the other soloed tracks off automatically.  That way you can hear any Midi track at any given time using just two Main channels (right and left stereo).
 
Got to go run the garbage out to the street for the garbage persons; hope I helped

that's why I want a sound card with a lot of independent channels -- one for each staff

 
 
?
Not sure I follow this. Lots of independent input channels on a sound card are useful if you are using lots of microphones to INPUT recording data into Sonar to keep the recorded signals seperate so that they can be processed seperately in the computer. But if you are generating your sound in the box using softsynths, Sonar supplies the multiple channels/tracks to keep the mix seperated for tweaking. You can have essentially unlimited channels for mixing, not just one per staff but several per "instrument". From dozens of tracks that you work on seperately you mix down to the ones you need for output. Output of stereo only requires two outputs one for each ear. A single stereo plug output would be sufficient for that. Output of surround generally only requires five channels max, and they do not have to be speaker connections to the sound card if it has a digitally encoded output. The surround signal can be split in an external amp or speaker set from one connection and distributed to the multiple speakers. Even if you are controlling external synths, a midi out will allow daisy chaining of multiple devices.
 
The sound card for computer generated sound does not have to be very complicated at all. You want one that will deliver a clean (low noise) analogue output for your own listening, because it makes it easier to hear what you are doing. But even a crappy noisy card will not affect the quality of your digital music data in the computer, saved to disc or written to a CD. That data remains pristine, and is completely independent of the sound card quality.
 
If you are doing live recording, then the digital sound depends a lot on the quality of the room, the sound card, and the microphones and pre-amps you are using aliong the chain from analogue sound to digital recording, since the ability of a computer to clean up bad audio is limited once it has been converted to digital. And as mentioned previously you need one input/microphone channel for every analogue source you intend to record simultaneously.

@Slartabartfast:

I have been using an extremely ancient version of Sonar (3.0 if you must know).  If I want to hear all of the tracks while editing the MIDI file imported from a score-writing program, I have to send each track to a separate input channel of the sound card (a Soundblaster Audigy).  I had assumed that modern versions of Sonar worked the same way.

I have been using an extremely ancient version of Sonar (3.0 if you must know). If I want to hear all of the tracks while editing the MIDI file imported from a score-writing program, I have to send each track to a separate input channel of the sound card (a Soundblaster Audigy)

 
Well, it has been a while since I have used Sonar 3, but I believe that since Sonar 1 the number of tracks was unlimited. By assigning the output of multiple instrument tracks to the same output bus, then the output of that main bus to the sound card input, it was possible to hear the output of all of those tracks. Sonar 3 will cetainly let you assign multiple tracks output to a main bus, and I am pretty sure you could then assign the outputs of the mains to the inputs of the Creative Sound Mixer. I grant that the documentation for the Creative Sound Mixer was extremely limited, so it might not have been obvious how to do this, but I think your current setup should be able to sound multiple tracks simultaneously within the processing limits of your computer.  One difference between a pro or prosumer audio card and the soundblaster line, is that soundblasters had a built in synthesizer and soundfont player. So if you were using the soundblaster synth, you may have had some problems/limitations there.