go for the 2 GB card new games are starting to require more than 1 GB on the card. Example crysis 2 to run on ultra, DX11 and High res textures they recommend 1.5 GB. This will just Keep going up especially if you add high AA to it.
The 7850 might not be powerful enough in some situations to take advantage of more than 1gb of ram (So if the prices where further apart $20+ the 1gb could be good buy, but they are not)... But in your situation where you plan to Crossfire it, Than the 2gb is without a doubt the way to go.