Memory – Can I connect a different RAM to the existing one or do I have to replace it completely?

I've installed PC3-8500F (533 MHz) Samsung RAM (2 x 2 GB) on my HP Elitebook 8440p laptop. I would like to update it but the closest RAM that I have found is 204Pin PC3-8500 CL7 8c 128x8 DDR3-1066 SODIMM Samsung (2 x 2 GB).

This is proved by "memorystock.com". What should I do? Attach it to the installed RAM or replace it with a newer type memory, 8 GB What better deal?

Many websites recommend other products, but I have to ask. I do not expect my old laptop to be super fast after the upgrade, but that it will work 100% and that it will be useful.