I have an ASRock N68C-GS FX mobo that has both DDR2 and DDR3 supported (there are 2 slots for each RAM type). I can boot on 4 gb DDR2 ram, but when I put in my 8 GB DDR3, I cannot get it to post. The DDR3 is 2 4gb sticks, and the DDR2 is 2 2gb sticks. My OS is ubuntu 64 bit. Any help is appreciated
I'm sorry to hear you still had a problem. It sounds like either bad RAM (both sticks) or a faulty motherboard. Did you check the BIOS, though? I'm assuming it will automatically detect the RAM type but maybe there's a setting in there you can check (using the DDR2 that boots, of course). If you're able to, try to test those DDR3 in another computer.
But you might just get higher capacity DDR2 since you know that part works. There's a measurable performance difference but it's not huge.