Well, when I first got my Athlon XP 1700+ and my Soyo Dragon+ mobo, I was reluctant to use the onboard sound (c-media sound) because I wasnt sure of the sound quality and I thought that it was going to slow donw my computer. I'm very happy to have decided to give it a try because this onboard sound serve me well for 2 years and my friend, who got my board when I upgraded, is very pleased with. As for slowing down game, as long as you can get more than 30 frames per second, you will have nice gameplay, believe me! hundred and so FPS are for showing or benchmarking. The video card is much more important. I dont know on what system you play your games, but with today CPU, you have more than all the processing power you need. So give it a try, and decide after. I've been using onboard sound on my new mobo for 2 month and decided to upgrade because the quality is not good (worst than my other board) for listening MP3 and movies.
-Always put the blame on you first, then on the hardware !!!