You have to have a special motherboard to use PC1600 or PC2100. Chances are, your PIII-500 mobo can't handle it, but VIA makes a P3 motherboard that can. Honestly, though, PC1600 or PC2100 doesn't offer any advantage when paired with a Pentium III anyways. The PIII just can't take advantage of it.
Kelledin
bash-2.04$ kill -9 1
init: Just what do you think you're doing, Dave?