Basically, nothing additional is needed.
A couple of months ago, I swapped an X2 CPU into a system that had
previously had a single core (Newcastle) processor installed. XP Home SP2
detected the new CPU and automatically installed the new multi-processor
HAL. (XP home doesn't support multiple sockets, but it'll support a dual
core CPU in a single-socket mainboard.)
A new install of XP should work as well.
AMD offers some dual-core "drivers" for download, but all that they seem to
do is allow Windows to clock down the CPU when it is idle. That would be of
more interest on a laptop machine. I haven't tried them on my desktop.
Address scrambled. Replace nkbob with bobkn.