This is a single die card, meaning one GPU, just with 2GB of RAM. Now that's great and all, as it is with the GTXs having over 1GB(that's not shared), it's something we've been waiting for, for some time now.
Yet how that equates into real performance, is a different story. Reviews of the Fermi cards don't talk about it, and it's difficult for them to really pinpoint on that aspect. It's probably safe to say that the extra RAM (meaning over 1GB) does help, much like it helps with dual gpu solutions, such as the X2 or the 5970 or etc - although a bit better, as the card has full access to it.
The difference between a 5970 and a 5870 2GB, is probably going to end up being pretty much the same as the difference between a 5970 and a normal 5870 1GB version - although it has the potential to be faster IF and when such extra RAM is needed. But consider that the normal 5870 does well for average resolutions. It's just not a card for 2560 or greater resolution.
The real purpose of the 5870 2GB, is to provide a card that can push their Eyefinity. Whether it attracts additional normal users/buyers is just a bonus.
It's far from a replacement for the 5970.
If you were holding out thinking this was some new performance setter from ATi, then we'll be mistaken, unless of course multi-monitor is the route you're going. Yet then you have to ask yourself, why not just have a 5970? Price? Well we don't know for SURE yet what the 5870 2GB will weigh in at, and even if it is cheaper than the 5970..again why wouldn't you have one? What person spends potentially thousands on awesome monitors, for such a powerful setup, and then settles for anything less than the fastest card available? It's almost as if saying Nvidia releases a variant of the GTX 480, with 2.5GB of RAM and no other performance changes. If you were after the proper type of more 'beef' or grunt for your multi-monitor setup, wouldn't you achieve an overall better result from SLI 480s, rather than just one 480 with another 1GB of VRAM?
Again, maybe I've not followed this 2GB 5870 train closely enough, and am wrong...but if not, it seems like a very niche product from ATi.