Unless you game at native 30" resolutions you would not get anything out of 2gig over 1gig (at least not in todays games.. and by the time you would you would need dx11.. unless you want to game with full screen supersampled 8xAA...)
Yes, it is pretty nice.. if you enjoy rendering 145 152 000 extra pixels. Welcome to the slide show! lol
I know some people do insist on FS SS AA, I am not one and think that MSAA + adaptive AA is fine and dandy (at least at 1920*1200).
If you are one of them, well the memory will probably be required.. if not, then there is no point (unless it is really cheep) for anything less than 30" resolutions.
That being said, a 4870x2 will be faster than a 285 in most cases. The benchmarks for 1gig are pretty much exactly the same as 2gig for comparison of the two cards. As I said only at settings beyond your res would it make any difference (1-3%), or if you are sold on having to use crazy inefficient AA techniques. (to add, ati cards don't support supersampling anyway.. but why would I want to AA a solid texture.. )