From my understanding, Windows Vista and 7 both allocate large amounts of RAM namely for cache purposes. It'll give this up should a demanding app require it.Is it Aero that adds the extra resources that may be the reason, Windows 7 is perceived to require more RAM? If you disable it, then maybe it's more on par with XP or even makes better use of RAM?
If you really believe that superfetch makes things faster.. you are suckered into all the propaganda.windows 7 uses all your ram but that only makes it faster(google). it also looks better and is more responive. But i would get a 4670 or something before spending your money on an os.