1GB will be fine even for Photoshop - (* only circumstance it wouldn't be is if GPU acceleration was being used, in which case much better cards would be needed anyway)
They will be fine (but ignore the other poster, the GT610 would be fine as well)
You don't need SLI (or indeed want) in order to use 2 cards for 4 displays - install the cards, connect 2 monitors to each, and then run screens in independent mode (i.e. under Windows control panel - Multiple Displays: Extend desktop to this display). SLI is only needed for gaming - not multi displays.
Not sure why people keep suggesting a need for huge amounts of VRAM? The 2 cards is for 4 display outputs, nothing else. A 1080P 2D image uses about 8MB of VRAM. VRAM is only an issue when using 3D apps i.e. Games or GPU accelerated packages. Photoshop can be GPU accelerated, but not mentioned in the original post, otherwise a better GPU would be recommended.