OP - You do realize that VRAM is not combined - but mirrored. So whether you have two, three, four 2GB cards, it's only an equivalent of 2GB of VRAM spread across three screens. Naturally you'll have plenty of GPU power to push the FPS but it's still 2GB of VRAM not added together.
As VRAM in games are going in the direction of being more demanding, and alot of people already will debate that 1GB isn't enough to run one monitor (which I disagree but that's another subject) then I can't see how 2GB VRAM can be spread across three!
My advice if your ready to get your system up and running right now, nothing less than 3GB VRAM. Two or three GeForce GTX 580 3GB cards are the only 3GB flavor out at the moment. Hands down the best graphics card now across three monitors. You did say 'best'.
Now the next gen cards are coming out, being a realist and all the news I've read about being delayed with chips slow to production it's being speculated late Jan to mid Feb for the high end cards.
Rumor mill says the lower end cards will be out late December but I'll refrain from believing it until I see it. In your case you don't want low end cards so your back to the wait. If you can wait it out make your decision then you you'll have more options.
One of two things will happen in your decision process when the next gen cards come out.
A. You'll find that your going to have more choice for a bit more performance with less power consumption on the next gen cards and find a nice 3GB flavor that will suit your three screens, just not for less money. (If that even matters).
B. You'll see price drops on the current cards and we might learn something about the next gen cards that aren't more favorable than the current cards. You may end up getting the current cards anyway.
If your system needs to be built before January then your back down to the most logical decision. Two or three GeForce GTX 580 3GB cards. Prior to their release I'd say 2GB cards or a tri-fire 2GB VRAM would be better, no longer true. VRAM is king with multiple monitors.
Basically he confirms what I've been saying on a single 1080p monitor whether you have 1.5GB VRAM or 3GB VRAM it doesn't matter for better performance.
Now throw in triple monitors and try to play a game, the 2GB VRAM card will be beat substantially by the 3GB VRAM cards. Especially across your huge resolution where memory will be spread thin.