In some tests 7790 performs and in others GTX660 does. But GTX 660 is better and fast. If you get GTX 660 then you get, TXAA(New movie standard Anti Aliasing solution by Nvidia), Phyx(you'll feel difference in Some games as compared to ati i.e Mafia 2, Batman Arkham City, Borderlands 2), Cuda(Parallel computation that will help you in video rendering). Here's some benchmarks of some games performed on 7790 vs GTX 660.
The GTX 660 is a nice card, but if you can get the Ti version, I'd defenitely recommend that. On the other hand the GTX 760 should perform even better and has more bandwith. It will be more futureproof and you can always go for a SLi setup if you feel like games doesn't run like you require. So, I'd recommend you to go for the 660 Ti, but if you can affort a little more, the GTX 760 will be a better choose in lomg terms.
To answer your latest question: 2 GB of Vram is defenitely needed if you want to play the latest games in 1080p. If you want to play on even higherdesolutions, I'd recommend you to even get a 3 GB model. 1 GB is too low unless you are planning to play on lower resolutions like 720p.
thanks but I have this deal with a friend of mine who works at this pc store, and he's willing to give me a big discount on one of these. the thing is, I can't find much info on this MSI gtx660OC, it's not a twin frozr version but it has higher clock than the stock 660. on the other hand, 7790OC seems behind it, but I don't have any comparable reviews, just info on paper and it looks like a gtx660 could be a bit better card in the long run.