A system with a stock 6950 and a mildly clocked i7 920 uses 295W from the wall and around 260W by themselves in Crysis.
http://www.anandtech.com/bench/GPU11/229
That usage goes up to 320W in Furmark (single threaded application, impossible load on the GPU). That's also measured from the socket, with a 85-90% efficient PSU, so around 280W output.
In real world scenarios you should never exceed the Furmark power requirements (it's not a real load scenario), unless you're planning on running Furmark and prime95 at the same time.
Now tell me again that a 6950 requires a branded PSU with 50A on the 12V rail. Because I'm pretty sure you could hook up two of these cards with no major issues.
I say you should probably get a 32-35A/12V if you're going with a single card and high overclock in mind or 50-55A/12V if going Crossfire with these cards.
And I agree with the above DM's post about getting the new cards at launch, they are the most valuable at that time.
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