Have you tried putting the 512 and another stick in? I'd advise using the 128 for test purposes. If so, does it start? What about if the sticks are in a different order?
I wouldn't doubt a PC using PC100 is capable of handling 512MB sticks, but it's possible as well as BandwidthHoggs suggestion of proprietary RAM. Just try using the 512 stick alongside another. Then if the PC starts, then once you run a few programs it crashes/gives you some form of error then you may have a dud 512MB stick there.
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