My media PC is Vista 64 and my gaming PC is Vista 32. I got it the wrong way round, but that was my fault as I clicked on the wrong option when ordering.
There are a few programs that Vista64 doesn't like, for instance they still haven't got 64 bit codecs working properly for WMC and Winzip plays up a bit. But there are alternative programs or workarounds.
What do you use your PC for? and what type of Vista licence and level are you looking to buy?
If you buy a retail version, your options are open as long as you only have it loaded on one PC at a time.