Sorry, forgot to give you a link to ORTHOS:
http://sp2004.fre3.com/beta/beta2.htm
Choose the "Large, in-place FFTs" test.
Your system should be just as stable at 5-5-5-18, if not more. You could try CPU-Z to see the recommended timings and voltage on the SPD tab.
http://www.cpuid.com/cpuz.php
Then just change the BIOS settings according to those values.
EDIT: It might take a couple of hours before you get an error, which would indicate either an unstably clocked CPU or bad RAM. If it's been running for more than 24 hours and still no errors, your system should be stable and problem-free.