The best chipset for PIII processors was the BX. It supports 256MB per SLOT, so server boards should support 1GB via 4 modules. Low density memory is required, I suggest you look to Crucial.com for this not-so-common stuff.
Any attempts to go with a chipset that supports higher density will result in a lower performance chipset, unless you go all the way up to the i840 chipsets with RDAM.
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