your 64bit OS will find most standard drivers (EX- touchpad, keyboard, chipset generic sound cards cd rom ect...) anything fancy like scanners, printers, good graphic cards, decent sound cards, tv-tuners, and wireless will need to find 64bit drivers. you can find them by going to the manufactures website.
when i experimented with vista 64bit i found if a 64bit driver was not available for vista, but there was one for xp, SOMETIMES that one works. (my epson printer anyways)
in my own research i found that going 64bit really provides jack in performance increase *currently* (i know this will start a debate) but there is no true 64bit processor on the market for home computing, plus most apps are designed for 32 bit environments. the only real benefit of a 64 bit OS is the security. and for the security you will give up compatibility.