Well, a few things to check. This is one reason that people say to put in a SINGLE stick of RAM, and using the ONBOARD video, try to boot the mobo + CPU + RAM on a cardboard (mobo) box, by shorting the power-switch pins with a screwdriver. Just to see if your main components POST.
I would recommend, removing all but one stick of RAM, and check the manual to see which slot(s) it should be in, and remove the GPU, and see if it POSTs. If it does not, then take the mobo back out of the case (yeah, I know, PITA), and try the cardboard box boot.
If that doesn't work, then you probably have a bad component somewhere. If it does boot, then try some further trouble-shooting, and try powering down, adding the GPU, and attempting to POST using the GPU.
Also, check the case, and make sure that the standoffs (you DID use standoffs, right?), are all in the right place(s). (Where the mobo has screw-down holes, generally. Maybe not at the far edge on the right when it's in the case.)
Edit: Also, important, check if the CPU fan is plugged into the "correct" CPU_FAN header. Many/most mobos will detect the fan RPM on that fan header, and if there is no RPM signal, they will shut the PC down for safety.
Edit: I would also check mobo reviews for that mobo on Amazon and Newegg; see if anyone else has had the same problem (possible model defect?).