a) if there's RAM available and there are no high-pri processes going on, Vista will use that RAM for any low-pri or maintenance processes. it's called optimization. don't sweat it.
b) if you fire up a high-pri process -- a game -- vista will kill or hold those other processes and recover the memory they are using. when you're in a game, there's LESS OS memory footprint than XP. don't sweat it. let go and let vista, or something.
if yer gonna bitch about vista, bitch about the shitty nvidia driver support. that's just embarrassing.