Hi guys i am new so please be gentle

I have just finished putting to together my custom PC specs below:

Intel Core i7 920
Palit GeForce GTX 275
Real Power M850
6GB Corsair XMS TR3XG1333C9 DRR3

So i double and triple check eveything cables, connectors etc.. I go to turn it on and it repeatably reboots after about 3 secs. No display on screen, i tried booting with only one slot of RAM still the same problem.

I would be so grateful if you guys could help me