How can defective ram cause complete blank screen? Shouldn't the bios information load and show some kind of information about bad ram?
Every time I've had bad RAM, it's been a complete fail to boot, absolutely no video or reaction at all—not even into BIOS. What makes me wonder is that you said that you tried booting without RAM and with single RAM modules, so it seems odd to me to think that it would be a RAM issue.
It sounds like it might be a motherboard issue to me, since changing the RAM mode allowed you to boot. However, I would agree that you should run Memtest to see if you've got a bar module.