Don't get a 512MB 4870... as new games come out in the next 6 months, the lower memory amount will start to tell.
At least get a 4870 1GB, or spend an extra tenner to get a GTX 260 216.
Stu, the memory (512Mb or 1gig) affects the resolution you're able to game at, which is restricted by your monitor size (less than 24" and 512Mb will suffice) Its the available clocks that actually process the graphical information, and these are the same in both the 521mb and 1 gig cards, so both are equally able to handle games now and in future so long as your resolution isnt too high (over 1900*1200)
Not according to ATI, also re your 4870x2 2gig card, technically speaking theyre misleading you, it is just 2x1 gig cards strapped together in a self built x-fire mode, so you do not get the benefit of 2 gigs, it may as well be 1 gig. Just like in x-fire, like my system has 2x512mb cards, I only have 512Mb memory avilable tho, not 1 gig