I find it interesting that you were able to actually fit DDR400 modules in a DDR2 slot...
8O
You're right though. The board is a P4 board with the 865 chipset...too old. C2D requires the 900-series chipset. You either have a dual-core P4 or a single-core P4 with HT.
So if you're trying to run a C2D on that, I wouldn't be surprised if it blew to bits. You're lucky your only issue so far is that your memory is a few MHz too low. Get a new board with the 945 or 965 chipset and DDR2 RAM. To be cheaper, get a supported processor like a P4.
BUT if you do happen to have a supported CPU, then you may have to manually set your RAM speed. I've had that problem before. Try removing one stick and check the speeds on that. If it's 400mhz, check the other stick by itself. If still 400mhz, then just manually set to 400 with both sticks and try that out. For me the problem was nothing more than date of purchase. Same brand, same timings, same voltage. Possibly some hardware spec that was different and caused it to fault but worked perfectly when I set it manually.