socket A mobo so looking at:
AMD Athlon (650 MHz - 1400 MHz)
AMD Athlon XP (1500+ - 3200+)
AMD Duron (600 MHz - 1800 MHz)
AMD Sempron (2000+ - 3300+) =versions for socket A Barton (130 nm),Thoroughbred B/Thorton (130 nm)
AMD Athlon MP (1000 MHz - 3000+)
(as found on wikipedia.org)
so the best would be a socket A sempron or athlon XP