With Core 2 Quad Q6600 CPU (65nm)
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.
I have heard that quad core processors don't work on libreboot yet. Not sure why though... Although intel ones don't work very well I think for a reason in particular... blobs/non-free firmware, etc...