Shifting From X86 To X64..

i hv been using xp x86. now when i have 4 gig ram... xp x86 (sp3) wud take up only 3 gig.. (1gig graphic..) so m planning to shift to vista ul x64 or windows 7 x64. so i wanted to ask.. wether... all the games which i am running here in xp x86.. will they run in x64.. ( means those setups and all and it will wrok properly.. cuzz m shifting only for gaming and making my pc recognize 4gig. and which one shud i chose...?? vista x64.. or 7 x64