Quick first impression on the Asus Xonar U7 USB sound card.
+ Sounds great, big improvement over onboard sound
+ Includes Dolby Home Theatre app
+ Is portable, use on multiple PCs
+ 7.1 surround
+ Good selection of ports
+ No power cable
+ Well priced
+ Includes SPDIF to TOSLINK adapter
- Needed to purchase a $2 RCA to 3.5mm adapter separately
Using the included Dolby EQ profiler set to Music (dynamic) and playing some 320kbps MP3 files I can easily hear a massive improvement on my office PC (which has some cheapo Gigabyte motherboard audio). And because it's USB, I can easily move it to other PCs. And at $99 it's considerably cheaper than some of their internal cards (I didn't have a spare slot for it in the office PC that I use for 90% (besides gaming and video editing)). ASUS claim that your typical cheap onboard sound has a SNR around 90db, whereas this is 114db. Remember that for every 3db increase, you are doubling the quality.
Only tiny downside is that you may need to purchase a L/R RCA to 3.5mm adapter. Unless you connect your PC to a TV or some other receive that uses stereo RCA. Can find adapters for around $2.
I'll run two USB cables from both PCs on my desk and can then quickly change which one is connected to the sound card. I'll use the onboard sound in the gaming PC to drive the buttkicker via SimVibe, and I'll keep a Toslink audio cable connected to my office PC as a backup in case I have the U7 connected elsewhere.
If you've got decent speakers/headphones but have been using onboard sound forever and enjoy music, I'd highly recommend this. You don't need to be an audiophile. If you have a $500+ gaming motherboard that advertises onboard sound with 110+ SNR then maybe don't bother.
ASUS Xonar U7 7.1 USB Sound Card Review