Jazzmail
King
I generally use my computers for games, including not only things like Civ IV but also games such as Fallout 3, the TES series, AoC, etc.
None of these games will support more than 2 GB.
Don't upgrade to Vista yet. Instead wait. Once you upgrade the hardware, get a new rig instead with Vista 64 installed (and the original CD). It's too early.
If you upgrade now you may discover that none of the games you play have improved and that some hardware isn't working because the vendors didn't make 64 bit drivers back then.