No game will normally use more than 2-3gb. your 8gb is plenty.
I agree that a single good card is better than dual cards. Dual cards may cause tearing or stuttering.
Your 750w psu can handle either cf 270 or up to a GTX780ti if you want.
Ignore specs as a selection criteria. What they are will be different per card and will work differently.
If you want one metric, buy the highest priced card you are willing to pay for.
You will get fair value at any price point. To see a significant difference, make it a biggish jump from the 270. I would think a GTX780 or R9-290 would be right. Much less, and you run the risk of being disappointed.
If you have some funds to spare, I suggest a SSD for the "C" drive.