I can tell you right now that ALL Pentium III motherboards used PC100 or PC133 SDRAM. DDR SDRAM will not work. Plus, it is physically keyed differently like dementg mentioned. There are two notches in regular SDRAM and only one notch in DDR. So DDR shouldn't even fit to begin with.
Because the PC is pretty outdated, I wouldn't bother trying to find the right brand or quality. At this point, look on
or eBay for a couple 128MB sticks. The max most PIII motherboards allowed was 1GB, though there are some that only went up to 512MB. Plus the biggest stick you can use in each slot will be either 128MB or 256MB on that motherboard. And if you have an older version of Windows such as 98 or ME, don't bother going over 512MB. It won't do you any good and could actually cause problems (see the FAQ section under my profile if you need an explanation).
Decent brands to go with: Micron (Crucial), Corsair, OCZ, Kingston, Samsung, etc.
Remember that it's not worth pumping much money into an upgrade. Every dollar over $50 is better used towards building a new one or even buying a new cheapie from Dell/HP/Gateway.
~cdogg
"
Insanity: doing the same thing over and over again and expecting different results." - Albert Einstein
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