Have you checked out any of the benches? I still recommend the GTS but if you want to be certain,
check out this thread for links to almost two dozen reviews. See how your favorite games fare on various hardware and how SLI scales on the GX2.
I don't have any preference between EVGA, XFX and BFG. Frequent upgraders tend to choose EVGA for the step-up, others like XFX's warranty (which transfers to the new owner if you sell it later) which supposedly adds to their resale value. I'd just go for whatever is cheaper/bundles a better game but that's just me.
I have seen tons of people running overclocked quad-core CPUs and 8800GTXs on the 520-watt Corsair 520HX power supply. At this point, I'd say go for a quality 550+ PSU if you are certain you will never run SLI/Crossfire. A quality 550W will handle an overclocked quad plus the latest-and-greatest single-GPU video card.