In short, it depends on which MB and CPU you are currently using, as there were/are many socket 775 dual cores, but, you will obviously not be using socket 1155/or 1156 socket cpus with them...
MBs supporting the latest i3/i5/i7 cpus are avail for socket 1155 (using H61/H67 chipsets) for as little as $80 US....