Real good... I was almost ready to give up on the big version of Ultimate if it didn't include the option for the higher-end 64-bit OS. After all, with today's hardware, why bother with the old, slow 32-bit? I'm surprised that 32-bit OSes weren't obsolete after Vista SP1 was released. I run Vista Ultimate x64 SP2 myself, and no troubles here, of course, it's better getting the full version rather than upgrading, in case there's other software that won't make the big switch, installing from scratch is usually better, anyway.
Ultimate comes with both 32 and 64 and it has one license number. So you can install either but not activate BOTH on that license. But tou can call Microsoft to buy an additional license number, let's say, if you have two computers, an old 32 bit and a newer 64. We tried the 32 bit version on a five yr old HP Pavilion with a Pentium 3 and it went very well.