It depends on the game, however many newer games ie. Arkham Origins, BF4 etc... They will use almost exactly 2GB of vram at ultra, sometimes even more. If your card doesn't have the vram, it will only use what it has, however if you do ultra settings in BF with a higher occlusion and more AA, you will need the extra vram. It's not just about resolution, here is a little bit outdated video from linus of linutechtips to explain the basics though:
http://www.youtube.com/watch?v=JBN5MNpYWlY
Also in this, is the specs of each gpu you are considering. Core architecture, architecture evolution as some chips are redesigned in later models, shading processors, memory bandwidth etc etc. Even all of these details and how they interact with your MB and other components. I would suggest in the current generation of cards, getting a 760 or above with at least 2gb of vram, though if it were me I'd chose 4 and personally I am going with the 770 4gb. The amount of shadders, for me at the price point is the seller. Also, clock speeds and such do matter sometimes, however mostly not so much. Quality over quantity. Look at AMD cpu's, they are decent no doubt, however per core against intel comparable cpu's they loose, because the single cores are usually 20-30% slower than one core of an intel. I bring that up as a close reference to gpu's. Also, the gpu is dependant like I said on what application you are using it for. Keep these tips in mind, and try not to listen to what every poster says on here, I have read these forums for quite some time now, and never posted, however I am tired of the misleading responses with uninformative lines of bable.
In the price point you seem to have, go with the 760 4GB for maybe 30 extra bucks, and I'm sure you will be pleased and future proofed.
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