rdouglass,
Thanks for the offer, but I think I'll pass since it's highly unlikely my memory is bad. You can save them for some other guy in need.
For the record, I have two 512Mb PC2700 333Mhz sticks. One is the original stick that came with my computer, and the second is one I just bought on July 2nd. Based on the test results and some tech forum references for my particular stop errors and failed memory tests seem to indicate a problem with the system timing. I've tried to boost the voltage slightly for the memory and fiddled with various combinations for the timing, but no luck yet. There is an observation with my board that the PCI frequency runs higher than what is indicated in BIOS; but I haven't figured out quite which way to adjust the settings to reduce it.
The stop errors are not constant or anything - but I'd like to eventually solve the problem.