It won’t make your games perform much better. I’d suggest upgrading your graphics card first, then if you’re still having issues overclock your CPU. Very few games are CPU limited in terms of performance, the only one that comes to mind now is Supreme Commander.
With CPU overclocking, you’ll need to buy an aftermarket CPU cooler since the stock cooler doesn’t do much. In most computer cases, having the case closed actually routes air better than having the case open because of some thermodynamics stuff. The cases are designed to have a proper air flow, or at least should be if you don’t have some cheap generic case.
With overclocking, don’t just ask for help here, read some guides. There’s tons of “newbie” guides online written by people who have been doing overclocking for many years.