What ever your upgrade, everyone seems to leave out the need for a New copy of Windows 7 / 8.... (so add in $100 to $150)
If you swap out CPU & Motherboard, ya can't reuse your existing copy of the operating system.... Microsoft's licensing won't allow it (keyed to those components..)
BTW, just to prove to yourself how little (or much) a CPU will impact gaming FPS (when you have a GTX780 !!!)
- UNder clock your current CPU to say 3Ghz
- then try 2Ghz
Run your game bench marks and not the FPS change, IF any......