You can check your CPU temperatures using a utility like HWMonitor...
http://www.cpuid.com/softwares/hwmonitor.html
The range for what constitutes a good temperature will vary depending on the processor though. You can check your load temperatures by running a CPU-demanding application at the same time. One example that's often used is Prime95's "torture test", since it can push a CPU's temperatures beyond what you would normally encounter with most other programs, allowing you to make sure a system will remain stable under extended loads...
http://files.extremeoverclocking.com/file.php?f=205