i hear that xp can only suppot 3 gigs and vista will run 4 is this true, and if i get the 64 bit version of vista will most pc games work on it ?

HiAs per my knowledge VISTA is a 32 bit O/S. i had installed my pc

From what I can find XP will support up to 4GB of Ram. Vista, depending on which version you get will either support 8GB, or 16GB of Ram. AS for the 64 bit version, in order to use that you'll need to get a computer with a 64 bit processor, not sure what games are supported, they don't say much about it, you'll have to do some searching for that.