Wouldn't I run into VRAM issues? I was thinking 4GB just in case this happens.
Not with 2 GPU's. With 2 GPU's you run into low FPS from a lack of GPU power/256 bit bus limitations and as such have to lower settings to increase performance which in turn drops the amount of VRAM being used as well. I'm not even talking "snobby" levels of low performance - you're looking at minimums of 20ish depending on the game at settings which bring you close to the VRAM limit.
So you drop some AA to get better performance (the nVidia 600 series cards can't really handle high levels of AA at triple screen resolution with enjoyable frame rates) and you drop the VRAM way down away from 2GB.
Everyone's favourite example - BF3:
Ultra preset with FXAA and no MSAA = ~1450-1550MB of VRAM.
2x MSAA = ~1750-1850MB
4x MSAA = ~ 1950-2020 (approx)
Note I never ran out of VRAM in BF3 64 MP. However at the 4x MSAA the game is frankly unplayable due to low FPS, borderline OK at 2x MSAA and butter smooth without any MSAA. I have experienced VRAM running out so can state categorically that it's OK.
The point is that at the settings which you'll be playing at to get decent FPS, you'll be nowhere near the VRAM limit anyway.
Comparatively, 7950 CF can max out BF3 with decent performance althought I still dropped 4x to 2x.
With 670/680's you need triple screen and 3 GPU's to benefit from the extra 2GB on 4GB cards.
I had triple screen and 680 SLI for a while so speaking from experience and endless testing

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