You won't see much of an increase if at all.
The only time changing or overclocking your CPU will give you a significant increase in FPS is if you're being bottlenecked by your CPU.
The i5 2310 is more than capable of keeping up with your R9 290.
If you get another R9 290 you might want to look at upgrading your CPU as well
Shazamy is right. You won't see much gain unless your game is CPU bound. This usually happens in some MMOs and a few modern games that will really load up all CPU cores.
Most of the time, your GPU is more important to gaming performance. Intel i5, i7, and AMD FX 8 core CPUs are plenty powerful to support dual GPUs. i5 and i7 have better raw per core performance, but AMD has more available PCIe lanes so it scales very well on a 990FX board with dual GPUs.