The cheapest and best OC'ing chip of AMD is tyhe Athlon XP1700+ with T-bredB core. Ypu can get those chips far in the 2 GHz with nice cooling.
An T-bredB XP2100+ is also very good in OC'ing but is slightly more expensive.
Both chips come unlocked with the T-bredB (or T-bredA but that one OCs not as high as the B) core.
My dual-PSU PC is so powerfull that the neighbourhood dims when I turn it on