Try this:
Assemble just the motherboard, CPU and heatsink and connect power supply to it (this can be done either inside or outside the case, I usually do this outside the case before i assemble for the first time)
Silence indicates an issue with any of the following:
PSU - in this case, not likely as you stated you are getting power
Motherboard - in this case, not likely as you have tried 2 different motherboards
CPU - Possible culprit?
A long series of beeps indicates a RAM issue (this is where you will want to add a stick of RAM and boot)
After adding a stick of RAM, if the beep pattern is one long beep followed by 2-3 short beeps, that RAM stick is good and you can remove it, set it aside and try another RAM stick. Do this for all RAM sticks.
Bad RAM will either yield silence or long single beeps.
If RAM is good, try the video card and connect it to a monitor to see if you get to the BIOS screen (usually one beep and then you should see the POST screen)
After that, try the hard drive.