I'd definitely increase the north bridge voltage. 4 DIMMs puts quite a bit of stress on the NB and usually requires a voltage bump. Though it's odd how it still fails with only 1 DIMM.
I've never used Vista's memory diagnostic though, always used
MemTest86+ though it doesn't have P45 support yet so you can't really rely on those results either.
Maybe try updating your BIOS too if you haven't already.