I would point to the CPU. When RAM fails (among many other things) the motherbard gives some beeps warning. You can mount the motherboard outside the case and turn it on by making contact on the power pins (with a screwdriver for example). First time you do it only with CPU to see what it does (should give beeps depending on what components it lacks) and then you go adding components one by one.

http://www.computerhope.com/beep.htm