32bit Vista will run just fine on a 64bit processor.
32bit Vista is limited to 3gb of useable memory. (If you see it with 4gb of memory, that's the tipoff that the memory is "shared" with an onboard video card...onboard video is not what you want, trust me.)
32bit applications cannot make full use of the 64bit or multi-core processor's power.

*64bit Vista has no theoretical memory limit. (Toss 8gb of fast memory on top of a Core I7 Extreme w/ dual 1gb NVidia Vids add a decent mainboard and you'll own the gaming rooms.)

Developers are finally updating applications to take advantage of the 64bit platform, multi-threading models and multi-core processors.

Unfortunately, by the time they catch-up we will all be running a 128bit OS.