Problems with four 1G sticks in AW9D Max

I've been running two 1G sticks in my AW9D Max with no issues (Corsair
XMS2 DDR2 800). When I added an additional two 1G sticks (same brand/
type), the board wouldn't post. I've cleared the CMOS and post was ok.
Now, Vista 64 bit won't always fully boot and sometimes locks up after
boot. My bios mem settings are 5-5-5-12. Voltage is default at 2.0v.
I've tried 1.9v with the same problem. Memory checks out ok. Latest
bios is loaded. Any ideas? TIA.