Originally Posted by juano
There isn't a whole lot of immediate need for more than a gigabyte of VRAM at your resolution, and by CFing your current card you would be able to hold out till there are better cards available. If you really just want to go overkill rather than need to go overkill (say if you had triple monitors or a 1600p screen) then you will be MUCH better off by getting a cheap upgrade now, and going big at the beginning of the next wave of GPUs. This is really much more sensible and effective than what you are considering. I'm a huge fan of VRAM and no fan of the current AMD GPUs, so if it weren't the wrong choice I'd love to tell you to go 3Gb 580 but in this case when you can get another one of your cards for so cheap just doing that is the much better choice.
this. at this resolution, unless ur planning to go higher, there really is no reason to spend an extra 450$ just to say you got a 3gb card.
even when games WILL require over 1gb, you will still be able to see great performance out of your 6870's.
and like juano said, you can always decide 6 months from now when the new line-up is out to upgrade to the newer cards. that's also a con for the 580 3gb (was considering it too up till a month or so ago) - it doesn't make that much sense to spend top dollar for a a card that's about to be dethroned..
check the anandtech benches for 6870 cf to make sure it's enough juice for you - http://www.anandtech.com/bench/Product/290?vs=301
i think that's PLENTY for current-gen games