When you power up the PC are you getting any POST beeps from the motherboard speaker? There isn't one built into the board so you have to make sure you attach one. If you are getting some beeps, what are they?
Have you tried building the PC outside of the case, on a cardboard box to eliminate the possibility of the motherboard shorting out? If not then this would be the best thing to do next.
Is your RAM one 4 Gig module or two 2 gig sticks? If it is 2 x 2 Gig sticks try removing one of the RAM modules from the motherboard and see if you can get it to boot.