Most people would recommend using Creative SoundBlaster cards (yukk),
eg. SB Live! 5.1 or SB Audigy
However,
I don't!
Most popular doesn't mean best quality.
Here's a list of top quality affordable sound cards:
M-Audio Revolution:
http://www.midiman.com/products/consumer/revolution_page1.php
Others:
Turtle Beach Santa Cruz (aka: "Videologic Sonic Fury" in Europe)
Hercules Fortissimo 2 or 3
Hercules GameTheater XP
NVidia NForce 2
Terratec DMX XFire 1024
Terratec Aureon Space 7.1
(use google.com search to find manufacturers' sites, or let me know if you want me to post direct url's)
If other people come along and mention SoundBlaster, read
this first.
Trust me, the last good card made by Creative without any serious issues was SB PCI 128, but these are a bit outdated now.