I'm adding 2GB of RAM to my desktop PC which is currently running 512MB, what I need to know is do I remove the 512 module and put the 2GB one in its place or can you run the both of them, teck guy at PC World said that if I leave the 512MB one in then the 2GB module will run at the lower speed of the 512 Module is this right?

I am using a PNY 2GB DDR2 800MHz PC2-640 OPTIMA DIMM if that is any help, I think that the existing 512MB module is a Infineon 64T64000HU3.7A DDR-SDRAM PC2-4300 (266MHz) - [DDR2-533]

All the product info is probably irrelevant but thought I would give it just in case.