All current PC cpu's are 64bits but these cpus can run 32 bit or 64bit software, like the Operating System.
If you are tyring to decide if you should buy 32bit windows or 64bit, get 64bit even if you will only have 4gb of memory (the max a 32bit OS can use)
If you already have a 32bit OS and are trying to decide which processor to get then Your choices are wide open.
-Enjoy-
Tim