If you're only going to overclock the 1.6a to a fsb of 133, then you don't need anymore than PC2100 ram(because that's what PC2100 ram is, 133 x 2 = 266mhz), but if you plan on going higher then you might want to get higher clocked ram like PC2400/2700 or above.
When overclocking a cpu you want to keep the least amount of components in your system overclocked, so you would go with PC2400 for a fsb of 150 or PC2700 for 166, and so on. That way if you have a mobo that will overclock the fsb that far, the cpu and mobo will be the factors for stability whereas the ram will not have to be overclocked.