I don't think there are any. DDR is obsolescent now, and Core2Duo CPUs are pretty recent. I've seen some low-quality models using a VIA chipset to run either DDR or DDR2 (both slots on board), but I wouldn't call a VIA chipset board "good quality".
I've found that DDR2 memory is pretty much on a par with DDR now for cost. Seeing as that's where you're hitting your problem, why don't you sell the DDR to offset against the price of some DDR2.
I was even more optimistic - I was originally looking for a mobo to help me upgrade to DDR2 and PCI-e, but able to support my s478 Pent4 chip! Needless to say I was woken up with a few slaps round the face!