Well, all Athlon XP's are socket A. So I picked a socket A processor. It will fit. Athlon XP's also use 184 pin DDR ram, so I chose PC3200 ram. It will downclock just fine even if your board only supports PC2100 or PC2700.
The only thing I'm uncertain about is the video card. You need to find what kind of slots you have available in your machine. The one I chose is AGP compatible. You need to see if you have an AGP slot, or if you only have PCI slots.
Ryan