Don't use USB sound cards. Not only is USB slower than onboard or PCI sound cards, it uses up CPU cycles to do everything (it has no onboard processing) and overall can lead to issues with compatibility, reliability, or just plain old bad quality.
EDIT: Sorry. to answer the original question: Yes, getting a real sound card will only help. Depending on your system you may or may not notice much of an improvement. Even an older sound card works wonders. I'm running an Audigy 1, and I hear the Audigy 2 is a very solid gamer card, even today with newer cards out.