If you have 4 go memory, you definitively need a 64 bit os to take advantage of it. As a lot of other people mentioned already, I would really stay away from Vista 32 or 64 bit. Don't get fooled by all the marketing around it, Vista is mainly eye candy and won't give you more performances (in fact, on the same computer, chances are, Vista will leave you a lot less resources for your programs than XP64)
So far all my programs work fine on XP64, and the only problem I had is the lack of driver for my scanner