yo the only thing that wouldnt work is some peripheral I.E Printers scanners digital cams
stuff i dont personaly use so im gonna go and upgrade immediatly the day i get my amd 64 bit athlon
I would like to add that some installers are still coded in 16bit. Microsoft has removed the 16Bit sub system from XP 64 so it can't run any 16bit apps. Just be aware of this when you go to download or install older software. Also I am not sure of the driver support in XP 64 so I would as some said above wait for longhorn.