For something with any halfway decent overclocking whatsoever for socket A, Biostar M7NCG-400. It uses the Nforce2 IGP chipset which doesn't officially support 400MHz FSB, but the board has chipset voltage adjustments so you can overclock it to that speed. The board will not let you traverse the 12.5 multiplier, so depending on your CPU default you can either have a range above it or below it, but not both. The rev2 board does not have the overclocking features, but rev1 and current rev7 does (no other known revisions). My experience has been with a rev1 board, the rev7 may be different. You won't find another mATX board of any make for any CPU with as much overclock-friendly options in BIOS.
I hope someone proves me wrong on that last statement. :thumbsup: