I think the cpu alone draws over 100w. Can I just lower the frequency? I would rather not buy another cpu. Unless there is a good low power am3+ one.
Here is a discussion about undervolting an FX-4100 that you may find helpful.
Lowering the voltage will definitely reduce power consumption and heat, and possibly extend the life of the CPU. But at some point a low voltage will cause errors in the CPU operations which could corrupt data or make the system unstable. Lower it in small steps and do some testing. If you plan to underclock and undervolt, only change one at a time.
If you don’t need much CPU grunt, you could look at the AM3 Athlons.
The Athlon II X2 250u has a 25W TDP.
You can significantly drop power draw by lowering clocks and voltages. At the same time you risk making the system unstable. You’ll have to rigorously stress test it to make sure it’s actually stable.