I have done a lot of research, and most people say a Core 2 Duo q6700, but some people say they have got a Core 2 Quad to wark (after the most recent BIOS update, that is) so, as long as you have updated the BIOS (and if you have the right cooler) you should be able to put a Core 2 Quad in there. I think it was the Core 2 Quad q6600, but don't take my word for it. 😛 Hope this helped!!