Thanks, everyone. I have done a little reading and the consensus seems to be that USB SQ sucks, so that option is out. BUT, I have discovered that my motherboard actually has S/PDIF coax and optical outs! It looks like I can hook up the AVR to those, but the 'Realtek HD Audio Manager' in the control panel makes it appear that I can't get sound simultaneously from both the analog 5.1 jacks and the S/PDIF...I don't really need simultaneous sound per se but it would make it easier to switch sound from desktop 5.1 to AVR 2.1. SO...I've also had the idea of running the AVR from the S/PDIF and the desktop 5.1 from the front headphone jack (I'm assuming the former would have better signal quality, and I want better SQ to the AVR). (Looks like the Realtek HD audio manager supports simultaneous audio from front headphone jack and rear connections).
At this point I only need to find something that will accept a single 2.5mm full range jack and output separate front, rear, center/sub to the desktop klipsch.