If you can keep the heat-generating devices cool, you bring down the ambient case temperature as well. The CPU in most cases is the biggest heat generator in your case, unless you have 7200RPM or faster hard drive and a scorching video card to boot. When overclocking your system, it is important to keep the heat-generating devices cool, which in turn will bring down your ambient case temperature. For example, if your video card likes to run around 60C, you'd better have proper ventilation in your case. If you can bring the temperature of your video card to under 40C then your case temperature will also be cooler as a result.
Hard drives are also a factor in high case temperature.
Keep in mind that most mainboards are only tested to 50C and when your whole system is baking without air flow, it is only a matter of time before instability, and ultimately, hardware failure are a fact. You should have an intake fan in the front, drawing air in, and an exhaust fan in the back. I have seen some designs of power supplies which exhaust the hot air into the PC instead of out the back, and also seen systems which have the rear fan pulling the hot power supply exhaust air back into the PC, and only one fan. A well-designed case will have 2 places for fans. The CPU is the most vital component in your PC, without it you can do nothing.