Offhand: On ultra at 1920x1200 (The dying breed of 16:10) my single 7970 can peak at using 2.3GB of the 3GB memory available on the card just in single player Battlefield 3, I haven't seen any indication of what memory usage multiplayer could have but can naturally assume just as much or more. With a reasonable overclock it's smooth as silk to play.
I don't know if crossfire will increase the total memory you have available, (ie. A couple of 2GB cards adds to 4GB of total memory or still just 2GB) but you'd PROBABLY want higher than 2GB card for plain old 1920x1080 if you want to turn all the eye candy on in high graphical fidelity titles.
I'm not sure about the memory use of the games you play, but just giving one example of a single game that can break the limits of 2GB memory cards and who knows what the future will hold for eyefinity enabled games.
But if you're unlikely to be playing these graphical blockbuster extraordinaire games then I don't know. Hopefully my little anecdote is helpful.