42 is fine. Especially without alot of case fans.
I would highly recommend getting a fan, attaching it to the case side lid (cutting a hole is required) and mounting it directly over the processor. Blowing some air right over it heatsink fan/chipset is amazing.
If youre really daring, cut a hole in the top of the case too, mount fan, and voila!
instant heat evacutation!!! <<<
I am amazed how few overclockers modify their cases to support more fans. One fan in front and two in the rear are simply not enough. If you plan on overclocking, try this!
For a Duron 700 that is not overclocked, 42 is too high. I have a duron 600@950 and I am around 42. You might want to make sure that you have proper contact with the heatsink/fan...
Also, I can tell you how many people I see improperly using thermal grease on a duron. Sometimes, thermal tape works better because of the high strength tension clips for socket a heatsink/fans...
I would get another cooler if all is normal. The vantec is excellent. 15 dollars. OR go for the Globalwin for 20-35
check here:
vantec cooler at 2cooltek
or
GlobalWIN