Most people would recommend something like RightMark CPU Clock Utility for displaying the CPU temperature and Prime95 for stress testing. Only a few people seem to know that Matthias Withopf has created two small but great applications.
For getting the temperature from the CPU, use CPUTempWatch. It will display the exact temperature for each core (read directly from the internal Core 2 temperature sensor) and display it quite nicely.
To stress test your CPU, use Core2MaxPerf. Why? Because 100% load in Task manager does not necessarily mean that your CPU is really loaded. That happens because the Core 2 can optimize a lot of instructions and thus 100% in Windows is not necessarily 100% for the CPU.
If you don't believe me, just start Prime95 and use the RightMark CPU Utility to display the CPU load compared with the OS load. Only if Core2MaxPerf is active, you will see that both fields contain 100%.
Don't wonder, Core2MaxPerf is in German but easy to operate. Just download it and execute it (Note: If you see a message box on Windows Vista, try to right-click the EXE file and select "Execute with administrative privileges"). The only thing you need to configure is on which cores it should be execute.
Core2MaxPerf will then open a new window with exactly one button "Abbrechen" (Cancel) which allows you to stop Core2MaxPerf. The simple reason: If you have selected all cores even Windows will have problems accepting your commands because of the CPU load. With that single button ("Abbrechen" = Cancel) you can simply press Enter and it stops so Windows will return to normal operation.
There is currently no better CPU burn-in test than this program for sure!