I'm looking to get a 670 for CUDA purposes but also do a bit of gaming on my three screens. I have 3 monitors of different resolutions. I know that the official documentation says that I need to have identical resolutions.
Currently my 6870 will reduce the resolutions of the bigger monitors when I enable Eyefinity; will surround do the same thing?
Yes it will, I would recommend getting the EVGA 4GB Superclocked card as you will need the 4GB VRAM for Surround View. I would also sell your monitors and get the same model for all 3 for less headaches.
How bad are we talking in terms of limitation? The only real games I play across the 3 screens are racing games like rFactor 2 / Project C.A.R.S that aren't so graphically intensive.
There will be very little limitation. The only reason to ever get a 4GB 670 is if you have them in sli. If you are running off of a single card, you will see very little difference in terms of performance. Even if you sli, the cards will still run great, but with a minor bottleneck.