go to athlonoc and download rc5 (they have a page for installing etc..). RC5 and Prime95 are pretty brutal on the system resources. Seti@home is also good but not as taxing as RC5 and Prime95.
It also helps to run either Seti@home or RC5 while running 3dmark2000 or Quake3 looped. That will tax the heck out of the system. If you have no crashes, your computer is stable and well burnt in.