If you're running DDR266, you should probably continue to run DDR266.
In addition to the DRAM frequency, try to match the "Latency parameters." Some laptops are pickier than others, and you might get away without matching, but at the same time you don't want to sacrifice either performance or stability.
Suggest running CPU-Z (http://www.cpuid.com/cpuz.php
) and paying particular attention to the SPD timings for the current memory chip(*) that you have installed.
From there, Newegg.com does a pretty job spelling the SPD numbers out, making for an easy purchase.
For what it'* worth.