GTX 770 vs R9 280X
Both GPUs are similar in performance , but the GTX 770 comes with 3 free games. Ac black flag , batman arkham origins , and splinter cell blacklist. While the newer R9 280x doesn't have any bundles. I'm sure AMD is going to add some deals to the R series cards.
So I would pick the GTX 770. No need for 4 GB. 2-3 GB is enough , unless you are using multiple monitors , or very high resolutions
The 7970GHz comes with free games too, and performs same as the 280X. But I'm not sure if he can get the free games over there.
To make a better decision maybe you can tell us more info:
Do you do video recording, streaming, encoding, 3D gaming?
What games you gona play? At what resolution?
Edit: If you SLI it won't double the effective vram
Get a 770
I suggest this really fast one from Palit: http://tinyurl.com/mwat5dl
Or the popular MSI Twin Frozr: http://tinyurl.com/m4eeml3
No need for an expensive 4GB one, won't need it for 1080p gaming. Some cards may be faster but they also cost more. Might as well overclock yourself it's free.