Daggas :
MART3R :
Tell your graphics card to run at full power:
To change this setting, with your mouse, right-click over the Windows desktop and select "NVIDIA Control Panel"
-> from the NVIDIA Control Panel, select the "Manage 3D settings" from the left column
-> click on the Power management mode drop down box and select "Prefer Maximum Performance"
You can also tell your cpu it is allowed to use full power by going into control panel under the Power Options.
*Both these tweaks are safe and are NOT overclocking*
If it still wont go above ~60% it is most likely because it is being bottlenecked by the CPU.
The solution would be to buy a new CPU in that case.
I built my little brother a computer with a G3258 Dual core CPU and a GTX 960 2GB and the GPU often gets bottlenecked in newer games.
One other option is that something in your computer is overheating, but that is rarely the case.
I hope it helps, good luck with your components.
Will intel Core i5-6400 work out?
Absolutely, the I5 6400 would work great and I doubt you would ever see your GPU get bottlenecked.
HOWEVER, your existing I3 CPU is LGA 1155, the same as some I5 3rd gens and I5 2nd gens.
With the information above, if I were you, I would find an I5 3rd or 2nd gen on Ebay by a trustworthy seller. Then check pcpartpicker.com and make sure your existing motherboard and the CPU you found is compatible.
If it is compatible youd be able to simply replace the CPU without changing the motherboard AND potentially save yourself some couple hundred dollars to buy something else for your PC, such as more ram or a 212 EVO CPU cooler to keep your new processor's temps cool.
Additional thoughts:
-Make sure your power supply would also be able to support your GPU and the new CPU (if it is 500w or more then it is more than enough.)
-Always skip through a benchmark video using the product youre about to buy before you buy it to make sure it performs well enough for you.