When looking for new sound cards you need to make sure you have the equipment to fully use the sound card. It is no good if you play $400 for a SBlive card which has 3D surround sound if you have only got 2 speakers. So bear in mind what you equipment you have.
Very broadly there are two kinds of sound cards around. 3D-sound cards and non-3D-sound cards
3D sound cards allow you to have a surround sound system for your PC. This gives your 360 degree sound so when you are playing a game you can pick out where the sound is, ie behind you and to the left. You need to have 4 speakers and a sub-woofer to fully take advantage of a 3d Sound card.
With non-3D cards they are more or less the same in quality, just make sure you read some reviews about sound cards before you buy any.
Sound Blaster 128bit
Creative's soundblaster range are the market dominators
they really are quite good.
Sound Blaster LIVE!
This gives the BEST surround sound quality.