This is just for other readers with the same problem.
Difference is the speed & bandwidth of the memory. The Ram that is stock in your laptop is an DDR2-533 (effective speed of the Front-side bus)/ PC2-4200 (with a bandwidth of 4,2Ghz/sec).
DDR2-533 RAM "fits" in a 266Mhz motherboard. (2x 266= ~533= (DDR)2).
The case is that Ram should be backward compatible, the fact is,..... it's always better to check. If it's not in the manual, just ask the manufacturer which RAM is compatible.
By the way, on http://www.crucial.com/ you'll find a tool that scans you pc and tells you which RAM you have, and need.