I'm the OP in this thread and you've all been supportive and helpful, so I thought I should report in.
 
To recap: The only "interface" I've ever used with my DAW is an ancient M-Audio Audiophile 2496 PCI card. It was the little brother to the Delta series, and it always worked for me, but it had limited I/O. Worse, it looked like there were not going to be any new drivers. I'm on Win 7. I don't even know if it will work on Windows 8, much less 10. So I went looking for a new interface.My biggest issue after audio quality was latency: I typically record electric guitar through an amp sim in the DAW, which means I have to monitor at the very end of the complete round trip -- through the mixer input, into the PC, through Sonar and the amp sim and back out to the monitors. With the old PCI card. the latency in this round trip was negligible. But apparently PCI interfaces are going away. Would a USB2 interface be fast enough for me?
 
I didn't understand most of what you all told me here. I was looking for somebody to say "This box here, for this price, will do what you want." Instead I got theory and numbers, which I am not good at. I don't blame you for not answering my specific question the way I wanted it answered. How could anybody else know what level of latency I could tolerate? In the end I figured I just had to work it out myself, face-to-face as it were, with a real interface. So I picked up the Focusrite Scarlett 6i6 (Note: I intend to use this with a laptop sometimes, so it had to be USB, not Firewire). I've only played with it for a couple of hours, and here's what I've found: It works fine! According to the readout in the Sonar Driver Settings screen it's a lot slower than my M-Audio card, but it's not bad enough to throw off my playing. Also, I have loaded the most complicated of my current projects and experimented with playing the soft synths that are already part of those projects, and got similar results -- the latency is within acceptable limits for me.
 
For those who might still be interested, here are some numbers. Looking at them, it seems that the PCI card was quite a bit "better" than the Scarlett, and yet I am not having a problem with this amount of latency. There must be something I'm missing about latency, I guess:
 
M-Audio Audiophile 2496 PCI card, sample rate 96k, 24 bit, buffer size 256 samples
     ASIO reported latencies (includes buffer and hardware latencies)
     Input: 3.1 msec.   299 samples
     Output: 3.0 msec.   284 samples
     Total Roundtrip: 6.1 msec.   583 samples
 
M-Audio Audiophile 2496 PCI card, sample rate 44.1k, 24 bit, buffer size 64 samples*
     ASIO reported latencies (includes buffer and hardware latencies)
     Input: 2.4 msec.   107 samples
     Output: 2.1 msec.   92 samples
     Total Roundtrip: 4.5 msec.   199 samples
 
Focusrite Scarlett 6i6, sample rate 44.1k, 24 bit, buffer size 4.0 msec.**
     ASIO reported latencies (includes buffer and hardware latencies)
     Input: 9.4 msec.   413 samples
     Output: 13.4 msec.   590 samples
     Total Roundtrip: 22.7 msec.   1003 samples
 
I always ran the M-Audio card at 96K, but before I installed the Scarlett I tested it at 44.1 because that seemed to be what most of you are doing. *For that particular test I lowered the buffer setting to 64 samples, although I didn't try to do any serious recording with these settings. I inserted an Addictive Drum track and noodled around on guitar, and there were no artifacts, so who knows? It might have been stable. Mainly I wanted to show everybody what the numbers looked like.
 
**On the Scarlett you can't set the buffer size in samples -- you have to use time. I started at 10 msec. and lowered it in steps until the latency was acceptable, which turned out to be 4.0 msec. I don't know how to translate that to samples, so I don't know how these settings compare.
 
General observations: My Schecter Blackjack with Seymore Duncan pickups breaks up the Scarlett's "Instrument" inputs, even with the pad. I have to use the "Line" setting in Mix Control. The knobs on this box are too small and too close to the edges of the fascia, thus too hard to see and use. I expect I'll get used to it, though, because -- and I hope I'm not imagining this -- everything sounds snappier through this box! I used to run the M-Audio card out through a mixer and then to the monitors. I boosted the bottom end a bit to compensate for my small nearfield speakers. Without this extra layer of electronics the sound is crisper and airier. I was never comfortable with the bass-boost kludge, but I felt I had to do something to keep myself from mixing everything with too much low end. My wife will be delighted to learn that I now need bigger speakers.
 
It's been a great learning experience, and it ain't over yet. Thanx everybody!

Something is fishy. That RT latency needs to be 10 or less for John(guitar) to be happy when recording on my 2i2.

jbraner
 On paper, your 22ms RT latency looks awfully high - but if if "feels" OK playing the guitar (and I'll be doing the exact same thing) then it sounds like you are all set ;-)

 

charlyg
Something is fishy. That RT latency needs to be 10 or less for John(guitar) to be happy when recording on my 2i2.

Guys, thanks for your observations, and you're both right. I think I may have been fooled by my own expectations. That is, I expected this box to be a lot worse than my old interface, and it wasn't, so I thought it was actually acceptable. But after I wrote my report I went back and played guitar some more and it was not acceptable. I changed my settings to this:

Focusrite Scarlett 6i6, sample rate 96k, 24 bit, buffer size 2.0 msec.
     ASIO reported latencies (includes buffer and hardware latencies)
     Input: 4.6 msec.   443 samples
     Output: 6.6 msec.   635 samples
     Total Roundtrip: 11.2 msec.   1078 samples
 
I can live with this. I increased the sample rate from 44.1 to 96 and reduced the buffer size from 4 msec. to 2 msec. The Sonar-reported RTL is now about double the 6.1 msec. I was getting with my M-Audio PCI card. The latency (at 11.2 msec.) feels decent while I'm playing, but I am still confused by the fact that I can't get it any better on paper. And I also don't know yet what it will be like on a project with a lot of soft synths and 20 audio tracks.
 
So for now I'm gonna say the 6i6 is usable for my purposes, but just barely. I have to push it to the maximum sample rate and almost the minimum buffer size to get it to perform half as well as my PCI card. It does sound better, and as long as nothing happens during projects to slow it down I should be OK. But those of you who need RTL<10 msec., this might not be the right box for you.
 
(charleyg, if you're reading, what are your settings?)

4.6
4.6
9.8 RT
24bit 48k 64 samples
 
Do you have the focusrite driver that gives you 3 settings? That is the newest driver, but it may still be in beta..
Anytime you don't have to go outside the PC is a good thing, response wise.

Larry did you check recently?
http://beta.focusrite.com/
Just updated a few days ago with new Beta Driver for Scarlett series

Larry, what is under the "Settings" button on your MixControl. Does that have a similar adjustment to my FireWire Driver Latency?

I was curious, since I have never seen the Scarlett version. With the numbers you are getting, this seems higher than expected to me. Have you tried the lowest setting (1msec) to see if you get any pops/crackles/dropouts?
 
As far as the "math," the easiest way to remember things is with units. Sample rate is samples/second, and your buffer is in milliseconds, so if you convert the buffer time to seconds, your units will cancel out to make sense.
 
Example: 44.1 = 44100 samples/sec and a buffer of 1msec = .001 seconds. Multiplying them gives units of "samples-seconds/seconds" and the "seconds" can be canceled top and bottom, so is just "samples." so in 1msec at 44.1 you would see 44100 samples/second * .001 second = 44.1 samples.
 
Similarly, 96000 samples/second * .0112 seconds (the 11.2msec RT you mentioned above) = 1075.2 samples (which you said was 1078, close enough since the RT is rounded off to 11.2).
 
Sample rate * time = samples (in that time)
Samples / sample rate = time
Samples / time = sample rate (this one is easy, samples/seconds = samples/second )
 
Just be sure the units used all match.

That is the correct stable driver. Mix Control 1.8 and the audio driver is 2.5.1,,  I just checked mine
And I get the about the same results at the same settings as you, I just tried that too. 
I think it's the hidden buffer issue as I don't "feel" any latency even when working at 44.1.  
 
My conclusion is unless you spend $500 or more your dealing with pour performance in RTL. All low end usb interfaces seem to perform about the same. But it is possible to get under 12 ms at the higher settings at risk of your system become on the edge. So you just have to raise and lower the buffer as you work. I leave mine on the 10ms setting 90% of the time for everything but messing with Guitar rig. And I'm not even using Guitar rig to record as I prefer my own pedals and set up, but it's a fun distraction form real work...