First thing to do is pull every card, including the video card, out of the computer, and disconnect every cable to the motherboard except for the power connection.

Then power up the system. If you hear beeps, look up the beep codes in the motherboard's manual (look online for the manual if you don't have a dead tree version).

If you don't hear beeps, scrounge an old "known-good" video card (a different one from the one that's in the system) and see if you can get it to boot that way.

If that doesn't work, you're looking at a bad mobo, bad CPU, or a bad power supply. If it were bad RAM, I'd expect to hear beeps.