Mac memory to upgrade laptops

Upgrading my laptop is an option I would prefer than buying a new one since my mac book is getting slower and slower due to so much applications I have to run on it, All I need is a Mac memory upgrade if I am correct. Usually computer memory is the one essential thing to speed up laptop processing many applications. I tried finding suitable Apple memory at first but I was told to limit my choice to a mac memory as this is the only thing which runs in mac book. I finally bought an upgrade of 4 GB of RAM so as of the moment, I am quite contented with the speed of my laptop and if ever in the future that it still is not enough, then will I purchase a newer and more powerful laptop in place of this one.