Any chance the monitor is dead or the video cable is not working? I realize you likely should get a single ("good") beep.
Strip it out of the case and do an minimal
Try an External build
(be sure to click "See full content" in JSC's original post), tear it down to a minimal functioning system, lay the main board (helps if a speaker connected to front header
or embedded on MB for beep codes; or if any diagnostic LEDs, LCD are present) on a piece of plain brown cardboard or other non-conducting surface with only
a Keyboard (plugged into a USB2 or PS/2 port), one stick of RAM (use a 1.5V stick), CPU, HSF, PSU and monitor to rule out shorts as well as bad components. [Temporarily use onboard graphics if available; otherwise use your video card.]
*Switch RAM stick and slot location. Keep seeing if your minimal system works.
*If stable, then add one component back to the system at a time. This will rule out components and maybe rule in the defective part. If you have any known working parts that can temporarily substitute this is a good time to try them one at a time.
*Often the mere act of pulling the system apart and reassembling it will make a good contact with a previously not completely seated part and result in a functioning system.
Assuming it is not a bad/missing connection and a minimal system is still exhibiting the same problems then:
1. I do not think it memory. You have tried 4 sticks with same problem. What are the odds?
2. You tried 2 MBs. What are the odds?
3. This leaves you with the PSU and CPU. I suppose also the keyboard. The keyboard is pretty easy to check if you have another one. And even if you don't you can try booting up without one and should see something on the screen from the BIOS.
The CPU is rarely bad from the factory. Assuming you did not put the CPU in the wrong orientation, I would suspect the PSU.
If you have a volt-ohm meter
you can test output voltages under a minor load. If you have even a minimal powered known working PSU it can power a minimal build.