And you plugged in the power connector to the GPU ?
Exactly which GPU do you have ...the one I looked at said it required a 450W PSU.
I have just made my self a new computer and everything went fine until l started it up what happens is that it starts up and all the fans start spinning and nothing come up on the screen and then after 5 seconds it will switch off and start up again. So I started unplugging cables inside the computer in an attempt to fix it and I unplugged the graphics card and the time between the computer booting then shutting down increased to about 15 seconds so I though it was the PSU the was not powerful enough to power my system but when I got a spare PSU which is 450W it still did not start with only two 120mm fans, the motherboard and a CPU plugged into to the PSU. So I now have no idea what is wrong any help would be great. I just wanted to say unlike similar questions I can not even get to the POST screen the screen is completely blank.
Intel i3-530 with stock cooler
Radeon HD 5770
400w corsair PSU
Seagate barracuda 7200.12
Two 120mm fans
If you need any more details please ask.