I have seen this issue before with some power supplies. Remember, it's not the overall power wattage of your supply that matters, it's how much power it supplies on each rail, versus what you are trying to provide on those rails. Your supply might have a total of 650 watts, but perhaps only 80 watts on a specific 12v rail that you are trying to suck down 120 watts of power from. That may cause the supply to shut itself off because it's being pushed too hard. Try spreading out the power distribution if you can.
Also, in VERY rare cases there are power supplies which are a little buggy when it comes to their automatic "Power Good" detection. A logic signal that lets the system know everything is doing ok power wise. I have seen some supplies have a laggy response on that line and cause the system to falsly shut off thinking there was an issue. Again though, this was RARE and usually on older supplies.
Agreed with Edwin. As he said, a lot of power supplies have a lot of certain "protections", such as over-voltage/under-voltage protection. Can you find out what the voltage is per rail? I am running a similar system (DP55KG, i7-860, 4GB memory and my main video card is based on nVidia 9600 chipset... single card in the 16x PCI express slot, no SLI).
May be simple, but if your power supply is one of these, make sure you are set to the proper voltage for your country... 115/120v for USA.. (you may know.. it's a red switch on the back of the power supply) -- if this is set to the higher voltage and you're pulling in 120v, you may be tripping the power supply's protection and it's shutting itself off. Simple, but I have seen it in the field on tech calls (and the person swore they didn't change the switch... those things are pretty hard to "accidentally" change... who knows though).
Also (don't take this as down-talk, I just don't know how much experience you have with computers) -- make sure you have the full 24-pin motherboard power connector in, *and* also (and maybe easier to miss if you're used to having a bit older computer): make sure you have TWO 4 pin power connectors (or an 8 pin connector) also powering the CPU.... for a total of 24+8 -- if you recall, older systems only required the 24+4 pin power connectors.
My last thought is that certain video cards can require extra power... and there is an additional SATA power connector on the bottom of the board, below the bottom PCI slot to provide extra power to it, which *may* be required if your video card is trying to pull too much power. I do *not* recommend just "trying" this to test it, you should do your homework first and find out how much power your card needs, and if it needs the additional SATA power connected to the board. This is all in the user's manual for further details.
For reference, if you are able to find it, here are my power supply voltage specs if you woud like to compare:
+3.3v rail and the +5v rail can put out 165w (shared between the two)... the +3.3v at 28amp and +5v at 25amp
+12v rail produces 600w/50amp output
As said.. i'm running the same mobo, processor... i am also running 3 SATA hard drives and two SATA DVD-RW drives and of course the PCI express video card (using PCI express 6-pin power connector). That is pretty much the "power eaters", at least the ones that pull enough power to matter. My power supply is rated at 700w, but can pull a bit more.
Hope that info helps... good luck with it. let us know if you find out what's wrong...