I dont think you understand. Its not bad for Vista using 70% of your ram. Franken PC elaborated on it a bit, but what superfetch is is a caching program. It figures out your computer habits, and what programs you use most often and caches them into ram. But when a program such as a game needs more than the 300mb of free ram, superfetch adjusts to the request and cuts down to give the program whatever ram it needs.
Play a game noting your ram usage before you launch it, then take a look when you exit the game.. With my computer it is around 60% when i launch and is around 20~30% after i exit BF2..
Vista is much better at managing ram then XP is; with XP if its not free it will never be, with vista if its not free it can be with a click of the mouse..
Hope that straightens things out for you.
Oh and heres some ram you might consider.
Heres a 2GB geil kit if you want to go for 3GB for $90
Or heres a 1GB kit to go with your existing kit