I think you're confusing a lot of issues. It doesn't matter what type of memory is on the graphics card, you can use a DDR3 graphics card on a PC which has mainboard RAM that is DDR2.
If your sig is to be believed you are already doing that, and on a PCI-E bus, which means you can run any modern graphics card.
So unless you have a reason to be asking for DDR2 graphics (which all but don't exist now), then you are really just asking for the best graphics card in that price range.