You already have an 865, which was only behind the 875. Many companies even brought over the PAT technology to the 865 so the difference between them was pretty much nothing. There isn't any more upgrading to be done if thats your board. It should support any S748 CPU out there.
The problem is all true dual cores for Intel are on the s775. You could find a hyperthreaded CPU for your board, but again you don't need to buy a new board for that. List your system specs, budget, and needs and we can see about getting you something.