i don't know how many times i have to tell people this, but Vista uses more memory than XP because of Superfetch. pre-loading the programs you use the most into memory allows that "free" memory to be useful for somthing even during idle. IMHO, that's a very useful feature, especially considering that it does it not just for apps like Firefox and Xfire, but i also noticed that its doing the same for some of my games that i play a lot like Oblivion and Sins of a Solar Empire.
and kudos that you're trying to play games on a laptop. out of curiosity, have you even tried Vista on a high end machine that's designed for gaming?? 50 FPS vs 60 FPS in Unreal III is hardly any difference at all, IMHO.
See the struggle of the faithless lot as they negate their time
How low to sink to the depths of their frame of mind