That'd be pushing your PSU, but I think you could manage it, yeah.
As far as the RAM goes, I don't think there will be a problem with adding another stick from a different brand, but make sure it's the same spec. Anything faster than your current RAM will have to slow down to match it.
edit: You'd have to upgrade your OS, though...32 bit operating systems can only recognize ~3GB or RAM. Anything more and you need 64 bit.
gotcha thanks man, going to have to get a "legit" copy of windows 7 then =].