Let me explain 2 ways of upgrading.
First. Cheaper way. You are not advised to overclock in this case due to cooling problems unless you drill the holes in the side panel to install extra fans, in this case I can just provide suggestions this it. So, let's continue. This way you have to use smaller CPU cooler to fit in original case and smaller fans. So once you spend money buying those parts and later if you decide to overclock you would have to buy new case, new cooler, larger one, get larger fans, so you will get stuck with extra parts. So, basically in the first choice you can get stronger CPU and cooled down without overclocking, this is considerably cheap and requires simpler work.
Second option. You want to overclock at the end. In this situation you would have to buy new case, buy larger, 120mm cooler and bunch of 120 mm fans, you would have to also remove motherboard backplate to secure larger, heavier CPU cooler, more money to spend, more work, more headaches and about 15% increase in performance.
So, which way you want to go? Read my "sticky" guide again. Once you choose the the way there is no turning back or switching the way due to the money already spent on parts.
So, make your choice.