Are they the same kind of RAM?
Have you tried each stick in each slot individually (four possible combinations)? Could be a faulty slot on the mobo.
If so, have you tried any other sticks of RAM in both slots at the same time? Could be a faulty controller/north bridge/whatever it is that controls those things on a mobo.
Yep, it are two sticks of G-Skill RipjawsX 4GB. I only got two slots on my mobo, so that won't be the problem. I got some old RAM here but it's 2x 2GB, I can try that. I only got no time to try it now, so I will let you know later.