64bit can definitely cause problems with software compatability. Printers especially seem to be bad in this regard. If you're not getting more then 4G of ram, pretty much don't bother with 64bit operating systems. If you are, carefully examine everything you plan on plugging into or running on the system to make sure you're going to be able to get drivers. There's also some software that just doesn't like it (the vpn software my campus uses won't work on 64 bit vista).