Software compatibility is less of a problem as 32 bit apps will run, DRIVER compatibility on some devices is more of an issue (Drivers can also include soft/virtual drivers for things like VPN).

If you plan to just use the on board devices that come with the system and just use mainly USB devices or newer PCI-Express devices then you are likely OK. Any legacy non-USB devices may not have 64 bit drivers in built and most vendors don't go back to old products to update them.

Any USB printer that needs all in one drivers (scanner/fax, etc) may also be a problem, standard USB printers that Vista has drivers for or most STANDARD non all in one HP printers are not a problem.