I switched back to 32bit Vista Ultimate from 64bit for one reason only, its impossible at the present time to get Coreavc codec to run in Vista Media center for .mkv files in x64, it does however work perfectly in Vista 32 using a small registry hack.
You can however get FFDshow to play your .mkv files in x64 but it isn't anywhere near as efficient as coreavc so i chose to go back to 32.
Apart from that problem which probably wont affect many people I would say go 64bit its deffinately a touch faster and at least it uses all available physical memory.