Even if your case doesn't have a ready-made mounting point, you should be able to find a spot to install a second fan. Depending on the type of connector your fan has, you can plug it into the motherboard fan connector or use one of the extra drive power cables for it's 12 volt supply. Leaving the case cover off is also a possible solution to overheating. Though not ascetically pleasing, it is a free solution that many overclockers employ. If your system crashes or seems to become unstable after a few minutes of operation, you may find that heat build-up is the problem.
Many vendors offer cooling packages with heatsinks and one, two or even three fans.
While these expensive cooling systems work very well, most people find that a simple heatsink setup with one or two ball-bearing fans will provide all the heat dissipation that your CPU needs. Be sure to remove all of the original thermal tape from the CPU if you're replacing an old heatsink. A little goes a long way since you're only trying to eliminate air gaps, not frost a cake. Too much is worse than not enough. Some interface materials are conductive so you'll want to be careful not to get any on the CPU pins or circuit board.