Sound card recomendations are always tough, because everyone hears something slightly different then the next guy. The cards already mentioned above are all great cards but the question is what do you want your sound card to do. Are you primarily a gamer? Then you may consider a card that supports EAX which means either a Creative card or an Azuentech. Do you want something for your HTPC? Then the ASUS recommended by Zero is the way to go.
My personal recommendation is the Azuentech Forte, it does everything very well not spectacular but very good for the price. Problem is finding one, I can't seem to find them at NCIX. I picked mine up on sale for 99 dollars but they average 140 dollars. The Forte is a great gaming card that supports EAX even though most games don't use it anymore and it does a decent job reproducing sound for movies.
That's my two cents for what it's worth good luck finding the right card. There are alot of folks on hear much better versed in sound technology that will weigh in over the next bit.
Auzentech, Inc. X-Fi Forte PCI-Express Soundcard : World First soundcards for Music, HTPC, and Gaming