There are no Core Duos for Socket 775 to my knowledge; they all use Socket 478/479/M. Either you'll buy a Core 2 Duo, or you'll fall back onto the Pentium D, etc.
EDIT: There are two exceptions I could think of: Buy a Socket 479 desktop board. You could use a Core Duo, but not a Core 2 Duo. Buy a Socket M desktop board. Either a Core/Core 2 Duo could be used. I have no idea where you will find a Socket M desktop board though.