adding a case fan...

Is it a bad idea to try adding a case fan...? I've done a fair bit of metalworking before and I was thinking of cutting a hole in my case and mounting a fan in it to keep it from getting too hot. I don't turn my computer off very often so I don't want it to heat up too much.. I also wouldn't mind trying overclocking it to be able to play some games better.. i've read about it before and saw a friend doing it...