Congrats on your new GPU, the 660 is a great chip :)
In concerns the the i5 you wish for, you should know that your current CPU uses a PLGA775 socket and the i5 uses a LGA1155 socket.
What this means to you is it has the wrong "plug-in" for your motherboard and is incompatible. If you'd want the i5 then you would also need a new motherboard with the LGA1155 socket :(
In this link
all the CPU's that have the socket PLGA775 are compatible with your motherboard and RAM. If you don't want to buy a new motherboard and RAM, these amount to your options.
Unfortunately, none of these CPUs will do your GPU justice. Also, I dont know what power supply you have but maybe a motherboard and cpu replacement alongside your high-powered 660 would need a power supply upgrade as well. Everything is tied together and makes things not so simple to change around.
For now I would recomment setting shadows to the highest settings. I read somewhere that this forces the GPU to do more of the rendering and might give you a performance increase.