Well,if you use Windows XP, the way I use it, I'd recommend more the sound card. It helps performance, trust me. I had a game with a choppy sound and graphics. As soon as I slapped in my SoundBlaster Live! sound card and reinstalled Windows XP...it was flying fast...and still is up to this date.
If you can spare for both, a single cheap sound card will do, and use the rest for the RAM.
If you can spare for both, a single cheap sound card will do, and use the rest for the RAM.