1) The 2gb version is probably worth it for triple monitors. I don't think I would pay the price premium for the MSI coolers.
I think I might go ahead and pay a bit more for a 2gb 6970. You may never need a second one for eyefinity. By the time you do, the next generation of cards should be out.
2) The cards require 2 6 pin pci-e power connectors, and most 750w psu's have 4. You should be OK.
3) Ram is cheap. It is best to get all your ram in one kit to insure compatibility.
Here is a study on some benefits of 8gb ram: http://blog.corsair.com/?p=65
Why get a P67 based motherboard when Z68 is not more expensive?
I like coolers with larger 120mm fans. 92mm fans are not as efficient, and can be noisy when they spin up under load.
The hyper 212 is only $10 more. There are others in that price range.
What will you use for a Hard drive?
A 1tb drive will be a bit faster, even if you don't need the capacity.
WD black or Samsung spinpoint are good.
If you can find the budget for it, include a ssd of some sort. It will make everything feel so much quicker.
Expect to pay $100 for a 40gb Intel 320 drive. Use it for the OS, or as a cache for a larger HDD.
Check out that combo, add the 6950 (see if you could get the 6970/570). And yes the combos 630w psu is enough for a single 6950/6970 and a 2500k and includes headroom for overclocking. Actually 630w is enough for crossfire 6950's Except the included psu doesnt have enough connectors but o well
6950 200w tdp
2500k 95w tdp
Z68 7w tdp
120mm fan 1w-2w per fan
Call it 400w with one 6950 600w with two.