Something you can try would be to open the side of your case and blow a regular house fan in the side. If you see the temperature decrease significantly, then your case probably isn't getting enough circulation so the ambient temperature of the air used to cool the CPU is high, and therefore not cooling the CPU efficiently.
As far as cooling fans, your case seems like it would be "adequate" for a duron, though adding additional case fans to increase air flow could help a lot. The fan on your northbridge chip is only going to be able to cool that chip itself and doesn't provide airflow through the case, same as the CPU fan/heatsink.
If opening the case and blowing a fan inside helps keep the processor running cooler, I'd suggest looking into at least a pci slot fan, or a fan that can fit into one of your 5 1/4" drive bays. You can also get modified cases as well, but some of them can get pricy.www.coolerguys.com
is a good spot for various things like that.