what happens if you try booting without the HDD and optical drive connected?
I tried that and have the same issue. Computer boots for about 5 seconds and then shuts off. Repeats over and over. No beeps.
As has been stated, remove all add on cards, boot with one stick of ram. Try to use onboard video if you are able, if not, try to find a different video card to boot from. Get into bios and see if you can last in there at least.
If that works, continue on with a hard drive and/or optical drive at most. Then install your OS, if that is all kosher install one add in card at a time.
You could have funky mobo/cpu/power supply if you cant stay stable in bios with just one stick of ram in. (try other sticks if the one stick doesnt work)
I'm using onboard graphics. My monitor doesn't even flash when I try to start the computer. It stays black. I've also tried a million combinations of the RAM I have and can't seem to get it to work. I'm going to get some new RAM and see if that fixes the problem.