266Mhz is the maximum clock speed for the ram.
the PCxxxx rating is the theoretical speed of the ram in Mbps, for 266Mhz ram it's actually PC2100.

In theory, any system that uses SDRAM can use memory that is rated faster than the requirements of the motherboard, as well as memory of the correct speed. So you can get faster ram than you need & it'll work fine (in theory)