This may seem like a silly question, but your not overclocking your
CPU?
Have a look at this page for a replacement heatsink for your
CPU. These fans are designed to get the heat away from your
CPU in the most efficent way possible. They normally have variable speed controls attached, but do cost a little more than the standard heatsinks that are supplied with the
CPU. They really are amazing, and well worth the money.
Quiet PC USA: Browsing AMD Athlon 64 / FX / Opteron - Socket 754 / 939 / 940
Also, at the back of your case, there is normally a small section where you can attach one of these:
Quiet PC USA: Browsing AcoustiFan Noiseless Computer Case Fan - 92mm