With that motherboard you can use PC100 or PC133 SDRAM on the two slots
provided for that - as you probably are now - or PC1600 or PC2100 DDR.
Your Duron processor can"t take advantage of the PC133 or PC2100, but will run
just fine with any of the four types.
If you were to upgrade to an XP+, you would need PC133, or better, PC2100
for optimum performance.