I purchased Home Premium, so I can say with certainty that it does not come with the 64-bit version. There's a card in the package that tells you how to obtain the CD or 64-bit version by mail for free. (Vista comes on a DVD, thus the option for ordering CD's.)
Like others have said, OEM versions are meant to be used on one machine for the life of the machine.
Here's another catch: If you buy the Upgrade of Home Premium (can't speak for Ultimate) you will not be able to just stick the DVD in the computer, boot from it and expect it to install Vista. It will not! It tells you to reboot into the existing Windows partition and start the Upgrade from there. The installer on Home Premium Upgrade will not repartition hard disks, even though it says it can if you boot from DVD. (A vicious circle, because you won't get far when booting from DVD.)
In my case, I was running WinXP Pro. The upgrade installer comes up and says that I cannot Upgrade from that version of Windows to Home Premium. (Literally). What it will do, however, is move your old "Windows", "Program Files" and "Documents and Settings" to a new folder called Windows.old. It will then do a clean install.
My old WinXP Pro installation with all my apps installed took around 6.5 GB. The new one (without the Windows.old folder) and the exact same apps (except a few that I haven't installed yet) consumes 23.6 GB.
Hopefully this helps you determine if you want Home Premium Upgrade or Ultimate. From what I hear, Ultimate doesn't have the upgrade and boot-from-DVD installation restrictions. (By the way, Home Premium Upgrade will upgrade a copy of Media Center. Just not WinXP Pro.)