well take a little look on this chart,
http://www.tomshardware.co.uk/charts/2011-gaming-graphics-charts/Metro-2033-Enthusiast,2664.html
You can see that the reference 6950's (lowest possible AMD cards to render at >30fps) in this title show that the 1GB card has BETTER performance than the reference 2GB model.
Also, the top of the heap is the GTX580 in SLI, with only 1.5GB of ram (each) versus the OVERCLOCKED 6990 with 4GB onboard.
This should illustrate to a fairly informed person that the capacity of the RAM onboard has very little influence over the performance in this case. I chose a very demanding title with very demanding settings to try and show as big a difference as possible.
In the 6990's favour, if you run a tri-monitor setup then you will need the additional RAM for the frame buffer at EXTREMELY high resolution. Nothing else will do. However, for anything that is running less than 2560 res it is just completely unnecessary.