Unplug EVERYTHING from the PSU except the mobo. Take out the video card and plug into onboard. Put in ONE stick of RAM, and try.
You should only have your CPU cooler and PSU plugged into the mobo, and not have the PSU connected to anything else. If it boots, get into the BIOS and check settings in there for video output, fan controls, etc then shut down and put another RAM stick in. Try to boot. Keep adding one component at a time, boot, repeat until everything is in or something fails.