s940 and AM2 are very different. The first being for servers and the later one for desktop.
s939 is a dead plataform for upgrading.
The absolute best for overclocing stability is the DFI LanParty NF590 SLI-M2R/G, wich runs at ~$205.
But if you want the most Mhz for your budget get the Epox EP-MF570SLI
would be your pick.