Well, last week I upgraded my A64 3700+ processor to a new X2 4800+. The 3700 ran great, but I wanted more juice to run Oracle 10g on my development server.
AMD Athlon 64 3700+ San Diego core (never overclocked) w/ retail heatsink - $80 shipped
Specifics: Socket 939, 2.2 GHz, 1 MB L2 cache...