I think a GTX660 would pair just fine with even a G620. You will see a performance hit in some games because of the CPU, but you get what you can get. A higher end graphics card will really start to be hindered due to CPU performance.
As far as graphics settings, it will depend on what game you are playing. BF4... probably decent settings, but definitely not Ultra. Same with COD. You will have to see what you can set it to. Your lower resolution will help a bit with better graphic settings.