My Socket A motherboard (an MSI KT6V) is used with the last of the XP Athlons (3200+) and DDR400 memory.
All were new in 2004, so you should be able to find a suitable case and mobo without much difficulty at quite low cost.
Try Gary Lindop, the owner of:
who has been a handy contact in recent years for such items.
I originally found his outlet on e-Bay.