Depends on what you wish to run. I was eager to go the 64bit route and ran Vista Ultimate first. The drivers for video just weren't there IMO and my games lagged a bunch. Also I couldn't get any of my CDROM emulation software to install nor run and I usually store ISO's of things like Linux, games, etc that I couldn't use. Benchmark software such as Aquamark and 3DMark06 also had issues with Aquamark not running at all and 06 I had to kinda beat to get to right right and at that it was slow benchies.
Again, all depends. Don't fear my stories if you have the time and the patience. I did all this in the course of 2 days roughly and had some time to do so.