Possibly the fb-dimm, did you tested with one stick of 1gb dual ranked 1.8v or the very rare 1.5v pc2 6400 fixed in auto ?
PCIE freq from my board won't boot above 101
1:1 in think is limited to max ~425-430 in quad channel
Your board is one of the last rev as me.
Easely checkable with the skull logo stamped on the southbridge cooler.
Rev 1 and 2 are only full black without logo.
Let's see how far we could go^^