I'd repectfully disagree entirely with the above post!
The difference in sound quality from your onboard sound card and a £100 (read $200) seperate card is practically non existant. The only part of the card that could make a difference will be the d-a convertors, and while you may be able to hear a difference between the two cards back to back, I very much doubt you could accuarately tell which was the better card. Neither would sound 'better', they'd sound slightly 'different', and even that would be extremely hard to tell.
Games nowadays are pretty impressive, and feature '3d sound' and all that other stuff. It does take more power to process, and freeing the cpu up by having a seperate soundcard will make a difference, especially if you have an older computer that already struggles with new games.