Four GB should be fine atm. Check out the latency timings for the RAM, if they're the same, go with the 1600. A higher RAM clock with higher CAS latencies defeats the purpose. And as far as L2 cache goes, that's usually something that gaming taxes pretty hard so it may be a factor.
It's going to be really power hungry when you play, so keep that in mind. Also you'll have to pay attention to cooling it while stressing the hardware, so if it's going to be on your lap, maybe get one of those special cooling mats as well.
And yes, definitely get the upgraded GPU.
Also, with all that hardware, you better be running a SSD.