the main difference is that the 64 bit os needs a 64 bit pc.
the 32 bit xp pro will run on 32 or 64 bit pcs
its said that 64 bit apps will run faster on this 64 bit os than comparable 32 bit apps and pcs..........we'll have to wait and see.
you may have to wait some time before 64 bit drivers are available for all your peripherals.
av and firewalls for xp 64 are still in short supply.
hardware
prices 
