'Under $300?' !!?!?
You can get an Audigy 2 Platinum Pro for about $250 at most.
If you're willing to spend that much, that's what I would suggest.
But if you don't have digital 5.1 speakers to go with it, there isn't much point in changing the sound card to something so expensive as an Audigy (2).
So, in case you need suggestions on good speakers, too:
Creative sound cards + Logitech/Klipsch speakers pwn all.