You have to get the 64 bit OEM version to get 64-bit.
Since my last post, I read an article from toms hardware, that compared video games on XP and Vista. And in Vista, every game took a performance hit. I beleive it was F.E.A.R or something, that took a 8.9% decrease in performance.
If I were you, stick with XP until their are a good amount of DX10 games out, because at the moment, unless you're building a computer and need to buy an OS anyway, theirs no reason to upgrade to Vista at this point in time.
Back on being mobile - ASUS G551- 15.3" 1080p - Core i7 4710HQ - GTX 860M - 16GB DDR3 1600 - 512GB Samsung 850 Pro - Steam ID: TheOtis