If you are overclocking your system then adding a case fan will NOT do much good for cooling of the CPU and RAM etc.
Adding the case fan is a good idea though, just don't expect it to solve all your needs.
Think about buying a good CPU cooling system (a large heat sink and fast, reliable fan). Also RAM coolers, HDD fans etc all help in cooling your system down alot more than a simple case fan will do.
Also remeber that case fans need to be positioned properly so that they work well with the other case fans you may have. Generally 1x120mm fan near the front to suck cool air into your case and either 2x80mm or 1x120mm fan on the back of the case to throw all the hot air out.
If all your fans face into the computer and blow air inside, then hot air will escape slowly and the temp will slowly rise.
Generally the more fans you have the better, as long as they are placed correctly and are up to the job, you should be ok. Just banging a fan on the side of your machine will probably not help your cooling and if you overclock the machine, you will certainly need to upgrade your heat sink and fan, especially as you mention that you rarely turn the machine off.