With Core 2 Quad Q6600 CPU (65nm)
2x 1GB : Doesn't boot
2x 2GB : Doesn't boot
1x 1GB : Boot
1x 2GB : Boot
With Core 2 Quad Q9650 or Xeon X5460 (45nm)
It boots fine with 1 or 2 sticks, no issues.
This seems to be fixed in coreboot as if I flash coreboot 4.6 or git, it boots fine with 2 sticks with my Q6600.