If you buy Ultimate, there are both 64 bit and a 32 bit DVDs in the box.
Otherwise, you can buy a 64 bit version of whichever flavor you want. If you have, or bought, a 32 bit version, Microsoft will send you a 64 bit CD for a shipping/handling charge. Something like 11 bucks.
As far as what is in which box: http://www.microsoft.com/windows/products/windowsvista/editions/choose.mspx
Home Premium is probably what most should get, though if you have your own favorite (non-Microsoft...) apps for pictures, movies, and whatnot then there's a bit less value in it for you.