Check if there are beep sounds when you start the cpu. Most motherboards give a beeping sound to indicate that the board has booted its part successfully, then its up to your operating system to continue booting the computer to windows.
If there are no beep sounds, re-attach the ram and video cards.
See if its possible to swap video cards.
The idea is to test by deduction..
1. From the motherboard, make sure that the basic hardware are the one's installed first - video card and a single ram stick
2. If step 1 boots successfully and a signal from the monitor appears, try using that same ram to boot from other ram slots. This will prove that all the ram slots are good. Then try adding the other ram sticks.
3. If 2 is good, connect now your hard disk, and other peripherals.
The idea is to test first with a different video card (if possible), and then deduce which part other than the video card is possibly causing the problem.
Goodluck and feel free to update us on your case