So I built my computer. I inserted the RAM different ways, I've inserted my graphics card right, the PSU is plugged in the 24pin motherboard connector, the cpu has the heatsink attached & connected. My HDD & lite-on dvd driver are connected to both the PSU & mobo, corresponding to the correct cables, my case fans are hooked up, everything is in correctly and I followed all of the manual instructions. But the computer won't start up. My specs - ATI Radeon 4890 MSI Motherboard, 790 GTX 3.0 GHz AMD Quad core 4 GB DDR3 RAM 750 GB HDD 550 W PSU [case] ATX Tower, x2 250mm fans on the side I'm thinking my PSU isn't powerful enough, because everything looks like it is assembled right, and all of the parts are compatible. Any help please? Thanks.

Did you plug the CPU Power Lead into the Motherboard as well as the 24 pin ATC connector ??

I found out what it was now, the 4 pin in the motherboard was unplugged, and I found out the PSU never came with it. Suffice to say, the computer won't work with out it. Thanks for the answer.