For pure sound quality in a consumer level card, M-Audio is a great choice. The M-Audio Revolution 5.1 is around $70 street price. It has real 24-bit/192khz output and hardware AC-3 for playing DVD's.
If you're a gamer, the Sound Blaster X-Fi will eat less processor in games than the M-Audio Revolution. The sound quality of the Revolution is slightly better, but the X-Fi is a decent card.
The Sound Blaster Live external USB is also very decent, and it works on a USB port.
Edit - Another note:
The Revolution is reported not to work in Vista (they do have beta drivers, so it's getting there), and the Sound Blaster's acceleration doesn't work in Vista. So if you're running Vista, most of what I said above isn't true.
[Edited by barleyguy on Sep 5, 2007 at 7:54 AM]