If you're looking for good sound, you might consider a higher-end sound card or audio rack that is proven for the task. I recommend thoroughly checking out objective reviews of hardware as well as subjective user reviews, since components used in a product don't tell everything about how the product performs.
I use this sound rack, which puts out excellent sound, but it's a bit outdated now, since it doesn't have a Vista driver (the XP functionality is very good, however):
WamiRack 24
After you pick an audio source, you'll need a power amp. If you want to be able to do surround-sound, i recommend a 6-channel amp, which will give full 5.1 support. I use the following amp, which has worked wonderfully:
Rane MA 6S
The third component you'll need will be speakers. This will probably be the most expensive of the three, but if you're careful, you can get some very nice speakers at a reasonable price. I use all Phase Technology speakers (they're great!)... models are PC subwoofer, two PC 3.1 fronts, two PC 6.1 surrounds. I map the center channel to the front speakers, to save space.
The above items are all a bit pricey if bought retail, but you can probably set yourself up nicely and affordably if you use Ebay, and are patient. That's where i got everything. PM me if you want assistance.
Edit: The sound source you pick should probably have at least 6 output channels, and you might want to match up the source with an amplifier as to whether they can use 'balanced' audio signals.
