You'd likely get higher FPS with the two 5970s. 2GB is still more than enough V-RAM anyway really. 4GB is overkill. Most games don't use over 1.5GB really, certainly not up to 4GB.So if I want better fps in higher resolution, Ishould go with 6990?
I can't remember if any 2GB 5970s were released, but looking at the way OP worded it and how they are "Black Edition" cards, I will guess that each one is 2GB. I think you're right though Mike as most 5970s were 1GB IIRC.
As the 6990 is two GPUs on a single PCB (and is thus not using Crossfire), I think that all 4GB is usable. I think it's only when you use more than one card that the V-RAM gets capped. Don't take my word for it, but I think that's how it works. 4GB is overkill anyway.
As I understand, it seems as if the two 5970 will have better FPS on 1080p while the 6990 will be slightly better on 2560x1600.
Assume there is only one way to really find out.
Test away!
Thanks for your inputs guys and girls