I don't know much about AMD CPU's and motherboards, but here is what I think is happening:
Overdrive is reporting correctly. It's doing the same thing CPUZ does. It is reporting a RAM frequency
of 400 Mhz instead of a RAM clock
of 800 Mhz.
Remember, DDR2 RAM transfers 2 chunks of data for each bus cycle (even if AMD technically doesn't use an FSB).
If you want to overclock, learn to use the BIOS.