I would get DDR3 board, boards that run DDR2 are labeled as "AM2+". Make sure that you get a board that supports X6s without a BIOS flash. To make sure you can buy a 8XX series board (chipset) and these all have support for the X6s without flashing the BIOS.
The only reason to get DDR2 board is if you have some DDR2 memory left over that you can use in a new build. Though I'm not sure if you'll find an AM2+ board that supports X6s without a BIOS flash.