A GTX 970 is a good option, since your processor is i5 I don't think you need to upgrade it any further, SLI 660s is a very good option too but I would actually go for the GTX 970 because 4GB VRAM is good for any resolution or multiple monitors and the CUDA on the card is very good too. It'll practically replace two 660s. By the way upgrading to SLI won't increase VRAM, it'll stay at 2gb. GTX 970 is a stupid good card for the price.