Here are the two I recommend: Prime95
- Most used downloaded software to stress your system to make sure it is stable throughout each process (overclocking... etc.). This is not a true benchmark program but it does test your system for stability and heat issues.
Futuremark's 3DMark Vantage (Vista / Win7)
or 3DMark06 (XP)
- Most used downloaded benchmark software to be compared against to see how your system ranks with similar parts.