Every so often, hardware makers strike out in a new direction, improving their product in an almost random manner — double the shader processors! stick two cards together! — and sometimes it actually has good results. The big move happening right now is a vast increase in the amount of RAM available to the video card, even though that’s not really what’s limiting game performance these days.
But if you trust HardOCP (and I do), that doubling of video RAM makes for a simply better gaming experience , and at a relatively small premium to boot (a 512MB 4870 costs ~$275, a 1GB 4870 ~$300). Although games are pushing more on the performance front than the RAM front, it helps to have that cushion of space for swapping, caching, and so on.