How futureproof is 6950 1GB Xfire

You know better than we do.

It depends on how picky you are, what your budget is.

For many a single 6850 is wonderful, giving decent gameplay.
CF 6850 is about the limits of reasonableness at 1080P
Some are not satisfied with sli GTX580.

Next year, the 28nm cards will launch, and many will become dissatisfied with their current cards.
But the old cards will not slow down, so they will still be as good as ever.

In general, I give a current generation of cards about two years before one will get the itch to replace them.
 
It doesn't go that way, and it depends on the game. And no one can predict what the upcoming games is carrying on, and the 1 GB of VRAM might be an issue for VRAM hungry games such as GTA IV, Crysis 2.

The higher the resolution in games, the more VRAM there should be since more textures need to be loaded. However some cards are not powerful enough to use the extra VRAM.

Due to technologies such as AA, MSAA, AF, QSCAA.. those really feed on the VRAM depending on the game.

Since you'll be gaming @ 1080P the HD 6950s should serve you quite well, at least for the current BF3, Crysis2, Metro 2033. Upcoming ACR, MW3 and some good games.