Yeah, you ever tried snapping a motherboard in half? A D12 is not going to phase it.
Can say beyond reasonable doubt that a HX650 will not have trouble with that card. Dare say that my GTX 260s would draw more in SLi, mated with an overclocked i7 920, I hardly drew over 500W when I plugged my PC into a wattmeter. Goes without saying that a quality PSU is a must, you have that covered.
Edit: Actually found the piece of paper I wrote the results on.
Full system load (meaning 100% CPU & GPU utilisation) = 550W
The only difference now is that I took that test when I was using an i7 940 that was clocked to 4.0GHz without Hyper-threading enabled, now I have a 920 at the same clock rate but with HT enabled meaning that I would be drawing a bit more now.