Rivals does say that 4GB RAM is the minimum, 6GB for GD adjusted and 8GB recommended.
List is here and here.
It also calls for a GTX 660 or higher which you have. I can't seem to find any good benchmarks for a 760 running need for speed to compare against.
I would say its the RAM but I am not 100% positive that the 760 is able to run everything on highest settings with out lag on intense scenes, I could be mistaken though. That 30 FPS cap seems to be a real pain.
Since it says 4GB is the minimum RAM you can kinda assume it also means its for running the lowest settings. You should go for 8GB since its currently the sweet spot for all games right now.