The fastest CPU made for LGA775 was the QX9770, which was a 1600FSB, 3.2GHz C2Q with fully unlocked mult. You likely won't find one for any reasonable price though, and many LGA775 mobos don't support it anyways, so I wouldn't bother in most cases. LGA775 is getting old enough that it really is just worth getting a new motherboard at this point and transitioning to something like P67.