ya i couldn't use my router when i had 64bit on my computer. you would also need 64 bit anti virus (i believe there's a free one..) in terms of performance, no it won't slow your games down or anything. i know steam has a 64 bit version of its games..(for counterstrike). you can run all the 32bit programs you want in 64bit windows.. just like how you can run 16bit programs on 32bit windows. you can run all your regular programs. as the other guy mentioned, some peripherals might not support it.. like if you get soem advanced usb mouse that you definitely need drivers for, then ya, make sure htey have 64bit drivers. but my creative soundcard, my 6600gt and other stuff all had 64bit supported drivers. only problem i recall having is my router.. and i think wireless mouse/keyboard as well.. i don't recall.
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius