I am currently looking into RAM clock speeds and cannot find any reference to the 1/3 or the 2/3 that comes after the 133 and the 266 MHz clock speeds.
Thanks in advance
That is just a reference ratio against your CPU base clock (or front side bus). 133 being 1:3 and 266 being 2:3 would mean your CPU base clock / FSB is 400 MHz.
Not certain if this applies to base clocks, but with older FSB systems you should use synchronous CPU and memory clocks for optimal performance (ratio 1:1).
Example: If you use the 1:3 ratio option, your memory is only synchronized to your processor at every 3 cycles. Where as 1:1 is in sync on EVERY cycle.