Your i5 2300 2.8 is your bottleneck more or less, as you stated, Nvidia will give you a higher FPS with the majority of games but they are also nearly double the price. I too have a Radeon HD7870 but have an i7 3770K which runs the game at about 60 FPS whilst recording in cities, 70-80 in all other instances. I would say personally, an OC is not even needed... Maybe just a new OC.
If you are not looking to pick up an i7, as it is not exactly needed unless you will be doing much rendering/graphical works, I would pick up a i5 3570k for about 220 bucks.
It would easily be cheaper than a new GPU and it also would 100% cure your problem, as a higher tier GPU will run you about 400-600 bucks and may not even be the problem.http://www.newegg.com/Product/Product.aspx?Item=N82E16819115233