the 6300 does benefit from overclocking regardless of the gpu used. on your system it will be perfectly happy to run along side the 760 even at stock.
i still recomend you oc the 6300 but as i said it will be fine and wont bottleneck at stock.
That PSU should be fine. I also agree with Hexit, you should really consider overclocking the 6300. Maybe not to the max but it should be fairly easy to hit 4.0 with it (but definitely get an aftermarket cooler)
Your PSU is enough for your system, in-game you will probably consume ~400 (maybe). The FX6300 is surprisingly a good performer in gaming. I myself have one overclocked from the stock 3.5Ghz to 4.2Ghz, I think for speeding up your PC in terms of opening programs etc then overclocking is something you should consider. The GTX is a good graphics card and there shouldn't be any bottlenecking going on between the two.
For your PSU you can calculate the total wattage used (consumed) by your system (click here)
The cooler your cpu runs the better, so yes the 212 EVO is worth it. Especially since on hot days your cpu will run hotter (unless you have air conditioning). Even if you do have air conditioning, the 212 EVO is pretty much the minimum CPU cooler that is considered by most overclockers since it is cheap and performs very well.