not doubting your searching, but i'm surprised you didn't see anything. Check the for sale threads sometimes when in doubt on equipment. If you see a repeat name, like griffin powermate, its a popular product.
As DP said, Creative's SB cards, both USB and PCMICA, are considered to be the best of the bunch. Audigy 2 NX is a popular USB card. I use solely usb audio cards right now(i like the ability to quickly switch between 5.1 surround sound and headphones and since the jack is right there to the steeo, my PPC has a hi fi go too

). Overall, hey perform wonderfully and i wouldn't hestiate getting one of them for the hell of it- esp since the NX comes with a remote control!
You can alter the bandwith used in your settings. Also, it only takes up 10%, via processor, trasnmeta, below 300mhz pentium, i'd worry, anythign else shoudl be fine