3Gb VRam card too limited for Arch Viz --> what soft/hardware options for Cycles ?

I am following some Arch Viz tuts, and my cycles render sessions keep on crashing on ‘not enough memory’. Have 2x 780Ti each 3Gb vram and a 590 (only 1.5Gb). I have set Cycles to only use 1 780Ti, as to use the 3Gb, instead of lowering all 3 cards to 1.5Gb. (Any news on cycles being able to 'unselect specific GPU’s ?)

What are my options

  • inside cycles / is there way of lowering vram requirements without lowering quality ? (Got advise to lower sub surfs but assume that would affect quality of result).
  • or should I go for the new GTX1080 - 8Gb !!

There are many versions of these GPU’s . I have tower with good cooling, but several GPU cards, so I thought the founder version might be better thant those 2 or 3 fan ACX versions - I was assuming the Founder version blows less hot air onto the other GPU’s. Anyone any experience with this ?