Do i need a Aftermarket CPU fan bearing in mind i am not thinking about overclocking but i will be playing high demand games on high graphics, i'l be using a 3.4ghz i5 Ivy bridge 4x core. Will i need a CPU fan considering i'm not looking to overclock yet? Also the case is a coolermaster silencio 650.
The stock fan will be fine at stock clock, albeit a little noisy. If you want to overclock, you'll need an aftermarket cooler. It may be simpler to get an aftermarket cooler off the get go. Depending on your case and choice in cooler, you may need to take most of your computer apart to stick a new cooler in there.
I would, more as a preventative measure than anything else. Most stock coolers are pieces of aluminum with a fan stuck to them.
Since the Silencio is a case made to reduce noise, that means that it will reduce air circulation as well. You can't have one without the other.
Keeping it in the coolermaster family, I recommend a CM 212 EVO @ $25. If you don't like that one, anything with a 90MM fan (minimum) and four heat pipes that make direct contact with your CPU. You literally spend as much as you like here, or as little, as long as you stay with those criteria.
That way you’re safe. Even if later you want to try that over clocking thing out