Case cooling is the most effective way to achieve all round lower temperatures, and it's quite easy too.
A normal computer case comes with one fan in it, and it is the one inside the power supply which normally is located at, or near the top of the case. First, make sure of the direction of the fan. We all know that hot air rise from the ground. Therefore, it is important that the fan inside the power supply blows the already heated air out of the case, not the other way around.
If it doesn't, and if you don't mind doing so, I would recommend flipping the fan so that the airflow goes in the right direction. Keep in mind that this procedure will void any warranty on that particular part.
If your power supply fan does suck air into the case, you will end up with air that has passed through the heatsink inside your power supply being blown into the top of your case - - and it stays there.
Since the hot air don't sink to the bottom of your case, all the hot air from the power supply and from all your other peripherals gets trapped in the top of your case, and your case temperature keep rising.
Next, you should get a second fan and put in the lower front of the case sucking cold air into it. I would recommend a big fan here; A 4,5" DC12V running at 5-6V provides you with all the airflow you need, plus it is super silent compared to its smaller counterparts. Doing these two things will guaranteed lower both your case temperatures and in turn CPU temperatures by several degrees.
Finally, two easy and absolutely free ways of improving the airflow inside your case are to get rid of what prevents it. IDE-, SCSI- and power-cables have a bad tendency to start floating around everywhere. My best advice is that you open your case and have a look inside.
Fig. 6 A tidy case improves your airflow.
If it looks like this, I would say you're on the safe side. If not, I would highly recommend that you think about cleaning up.
Fig. 7 Splitted UDMA33 cables.
Splitting the IDE cables is very easy, and free up a lot of space. A clean case is not only about aesthetics, it really help cooling your case.