2GB of VRAM is only really necessary at very high resolutions. Like above 1920x1080. Anything lower than that, a 1GB Radeon 6950 would be fine.
As for the PSU, The Radeon 6950 and 6870 could easily run off a quality branded 500W PSU. A quality branded 650-750W would allow for Crossfire for either cards. PSU requirements given by video card vendors are over-exaggerated. 750W-800W is just overkill and a waste of money for a single card setup. PSUs are most efficient when around 50% load and aren't efficient when below 20%. This leads to a higher electricity bill since the PC will be idle most of the time unless you're folding or hardcoe gaming. A 750W unit shouldn't even be considered unless there is a true intention of going Crossfire with those cards.
Perhaps you did not read the part (that you quoted) about overclocking? The "K" variety of CPU is made for it, and therefore, I would consider that, when looking at PSU options. Also, having more power there, for
when you upgrade (it is never a matter of "if") can only be a good thing.
And 2GB of VRAM
is necessary*, when you are using a lot of hi-res textures, at 1920x1080. I know this from direct experience.
* Or, as I said, perhaps 1.5GB would be enough. I use a 2GB graphics card, at "Full HD", and
almost always run over 1.5GB. It never
quite reaches the 2GB mark, but that is just as well! In other words, I am not posting based on theory, but on direct experience of the thing in question here.