Here's what I use, and I like it.
Audigy (first gen, with the firewire (nice bonus)). I am using linux with the Alsa emu10k1 drivers.
$69 OEM-style from a local computer store. Cheaper now, I got it when audigy2 first came out.
My speaker setup is a $300 home theater setup from Kmart.
Sony STR-K750P reciever. 5.1 Dobly/Pro-logic 1 and 2, DTS and all that happy horsesh*t.
The speakers that came with the setup would be best described as "extremely average". But they are small sattelite-style speakers and are easy to stick anywere.
1 4-speaker center, 2 2-speaker fronts, 1 subwoofer with it's own little 75 watt amp, 2 2-speaker rears (same speakers as front.)
75watts per channel.
With it all set up you can get pretty good sound. You can tell some of the lower midrange gets muffled and the subwoofer isa bit boomy at higher volumes. Good stereo imaging, though. (In multiplayer games you can realy tell the direction left-right were the other guy's foot steps are and transitions are smooth)
I got this because it was only a 50-75 dollars more expensive then the reciever alone, and Sony makes very clean sounding recievers and this will give me time to save up for some good speakers.
What a 5.1 gets you over a regular stereo is the ability to make sound more encompasing, basicly all it realy gets you is back speakers.
It won't improve music listening much over good quality stereo setup. What it's designed for is movie-style special effects, were you get very impressive sounds coming directly from the side or rear of your listening setup.
It's not capable of creating the subtle transitions and small directional changes front-to-back that you get from a nice stereo setup right-to-left. Even in movies that fully support the digital formats.
There are formats created for subtle front-back sound using setups with stuff like 3 speakers or some such nonsense. They used scientific theories on how the sound waves interact and reflect off of each other to create areas of different sounds. Very esoteric and hard to set up, sounds like crap if done incorrectly. Don't remember what it is called, though. Something that started developement in the 50's or something.
EDIT: here is what I was thinking of
Ambisonic
Then you have crazy new stuff like Holophonic Sound.
One of the big things that I noticed is that the correct "triangle" setup with stereo speakers is different with center speakers. You have to set the front speakers much farther apart and slighlty forward with them pointing towards you then you normally would do with just a 2 speaker set up. The 2 front stereo speakers are still the most important ones though.