All of them would be compatible with the A7A266, the PC1600,PC2100,PC2400 are just the ratings for the maximum speed it is speced to run at. You need PC2100 or faster DDR SDRAM to run your Athlon at 266MHz FSB, you can use PC1600 or faster for an Athlon at 200MHz FSB.
PC2400 is for those that are looking to overclock.
PC1600 DDR is usually capable of overclocking to PC2100 speeds.