Hey,
I guess it comes down to how much you want to spend.... and what for... if you want the best of the best then it'll cost you about £170 (ex VAT) for the Creative Audigy 2 (inc drive bay), £85 (ex VAT) (exc Drive bay)
*these prices are from Creative, it probably will be cheaper from elsewhere*
Creative Audigy 2
This product is not released for a few days yet, even when it is it'll be hard to get hard of at first.
Alternativly for a very good soundcard, you could get the Creative Audigy. This card is also good.
Also look at: Hercules Gamesurround Muse 5.1 DVD, Videologic Sonic Fury 6-channel, and phillips acustic edge PSC70617 5.1.
To get the most from these cards you will need a 5.1 speaker set up. Otherwise there isn't much point in buying a 5.1 sound card. For the 6.1 cards you will need a 6.1 speaker setup, for pc's only creative have these speakers.
I myself have the Creative Audigy platinum along with the inspire 5300 speakers. And it is loud and it is very good sound quality. but being the avid gamer i am i'll be upgrading to the Audigy2 ex when it is released next year.
I suggest you read around about the cards before you buy
Daz