Computer is constantly restarting

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Hi guys, this is my first post, and i'm at a loss as what i should do with my home-made computer.

Yesterday i bought the parts for a custom home-made computer, these parts being:

Intel Core i7 920 CPU
8GB (4x2GB) DDR3 OCZ Memory
1TB 7200rpm HDD
ATi Radeon HD 4870 x2 2GB
Gigabyte motherboard EX58-UD4
850W PSU

I've set everything up correctly, making sure everything is wired properly and no exposed ends are anywhere near the motherboard. Problem is, when i plug in the 8pin adapter on the motherboard, my computer is constantly restarting, when i turn it on the CPU Fan will spin, the LED's will come on, then 2 seconds later, it'll all go off, it'll then continue doing this untill i turn the PSU off at the back.
The wierd thing is, when i remove the 8-pin adapter, the computer will start up no problem, but i see nothing on my monitor at all. I also hear no HDD activity (there is no windows on the HDD, but surely i should hear some activity).
When i plug the 4-pin adapter in, the computer will no start at all.

I'm wondering if i have a powerfull enough PSU for my setup, and if i do, have i got a bad PSU? I've ruled out the graphics card and HDD as problems, and i've tried with only 1 stick of memory, but it still does the same thing.

I really dont know what to do here, it's not the first computer i've built, not by a long way, but this has me totally confused now.

Hopefully someone could possibly help me here!

Thanks a lot guys! :)
 
Usually anything from 450-watts and higher for most computers should do the trick, so there is the possibility of a faulty power supply.
 
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