yeah, this is pretty much for Chaos, but tag along if you want to pretend you're cool.
I'm going to be getting Vista, so I'll need more RAM. I have 2x1Gb, can I add another Gb by using 2X512 in the remaining 2 slots?
Yes - but watch for differences in speed (will work to slowest pair)
Matched pairs like chaos said.
Vista seems to not like differnet speed ram for reasons i cant fathom.. it crashes a lot... i tested it... xp was fine, vista was a bitch.. but that was waaaaay before sp1 so sit may have changed

was annoying cuz its quite clearly a problem for the mobo/cmos to sort out, not the os :X