Any program that installs drivers will likely not be compatible with 64-Bit
You need drivers for 64-Bit (32-Bit drivers don't work) so if your hardware is old you're probably out of luck.
16-Bit programs won't work (Older programs)
As for advantages, 64-Bit windows enables a faster communication with the memory, more RAM, and more security.
I have only two old pieces of hardware that I know of, one is the HP 1215 printer which I have the 64 bit drivers available in Windows 7 by using the drivers for the HP 7200 printer. The other was the 940c which is also available for use in the add printers section of printers in the control panel. Thankfully all my drivers were no problem.
Just keep in mind that Windows 7 Beta is just that.. a BETA so there will undoubtedly be some bugs, driver issues, incompatibilities of some kind no matter which version you use.. but yeah as Kyle said using the 64 bit allows you to use more ram and is more secure. It does aso make a slight difference (for the better) in the performance of some games.. But the main advantage right now with a 64 bit OS is the fact that you can use more RAM, which of course is always good!!