Yes...you can only take advantage of all GPU RAM when you pair them identically (512MB + 512MB, or 1GB + 1GB).
So both of these cards togther in CrossfireX mode will yield 1GB total in available RAM, but the upside is that
you get significant increases in fps across the board. Of course, those increases vary with driver version,
game engine, resolution, and IQ settings, but its safe to say that the performance is -almost- on par with a single 4870X2.