I assume by backround, you mean background programs? If I'm playing games I don't have any background programs running, they chew up system resources which would be better suited to running the game.
I'm using WinXP and even when I am playing MOHAA, I have around 25 process running (Ctrl-Alt-Del). I have already stopped most of them, using Start/Run/msconfig and the difference in performance is almost nothing. Obviously when playing I don't leave any program in the background (the ones you can switch to by using Alt-Tab).