|Replace the power supply, you can get a decent one for not that much more money than the fan. This one is $25. after the rebate:|
And a fan is what... $6. to $12. or more.
Install HWMonitor and get not only the temps but the fan speeds as well. A valuable tool in evaluating why you are getting hot.
If you want to know now if it is the lack of hot air exhaust from the power supply fan that is doing it (before even changing it), open the case and have a small portable fan blow air into your case. If it is still hot, you still will need to change or repair the power supply, but you have another problem. If the temps stabilize, then it is that your system relies heavily on the power supply's fan to keep your case cool and that may be enough to fix the problem. Personally, I would in this case, add or upgrade your rear case fan to improve air flow and keep it cool as well as replace the power supply.
If the system is on its last leg and you are planning on replacing it soon, do not spend too much on it, but the power supply I listed would be a good base for later building a decent machine as would decent case fan always be useful.
You have to be a little bit crazy to keep you from going insane.