It depends on what your looking for. By putting a dedicated sound card in, you won't see another 15FPS in your games. (well, maybe if you game at 640x480...) You will probably get perhaps as much as 5% more. What you will get is the option for better sound effects. From what I have come to understand in another thread, onboard sound is limited to EAX effect 2 or 3 depending on the motherboard. If you get a creative soundcard, you could move up to version 4 or 5. This allows for better positioning of objects. (you can tell where people are easier...)
The thing that bothers me is people who think they need a sound card, so they spend £80 on a soundcard, but buy a 7600GT because thats all the money they have left. If they drop that soundcard, they would have another $80 to spend on the video card, and can move up to a x1900xt/7900GT. This allows for much better IQ, while still providing 5.1+ sound.