depends on the card. Creative cards have a dedicated processor that handles the computations for positional sound and decoding duties. Most other cards do not, and rely on the codecs to be run on the cpu.
All of them give you better quality simply b/c the components are moved further away from the mobo traces.
I say all of this irrespective of brand favoritism. Creative has made some stinker cards, but the x-fi currently is very good. The powerful proc on it does alot of work. Sounds great for consumer level.
There are others that are good too, some better in the past than what they are now.
As for tangible performance gains... you would have to see raw benchmark numbers as I am not even sure you could perceive the diff by just playing a game... Buy one for the quality improvement, if you are after just performance your $ can be spent better elsewhere.