Currently the 64bit OS's (XP64 and Vista64) are a bit buggy and have a fair bit of trouble running 32 bit applications. In the case of Vista however, that is to be expected as it is still a 'pre-release' build.
For now I'd just stick to 32bit, and purchase a 64bit version of Vista when its officially and commercially released. By then hopefully it will be able to emulate and run every program known to humankind!
(64bit doesn't actually speed all that many programs up, and is only really beneficial if you have more than 4gb of RAM)
AMD 64 X2 4200+ @ 2.2ghz
2048mb Corsair PC3200 RAM (2x1gb)
WinFast NF4SK8AA Motherboard
660GB HDD Capacity (3xHDD)
600w Enermax PSU
eVga 8800 GTS 320mb