World of Warcraft: Cataclysm is running very slow and laggy?

I used to play WoW before Cataclysm came out and my computer ran Wrath of the Lich King great but right now before I go buy the new expansion I'm playing the original WoW 10 day free trial. But when I log on it runs very slow and my computer says it has 2.00gb of RAM. I used registry cleaners and cleaned out my registry, used disk cleaners and even used a site that tells you if your computer passes the requirements to play WoW. It told me I passed the requirements but it is still very laggy and slow. Do I have to buy more RAM? Others say it's my graphics card but Idk how to access my graphics card to see what kind it is. I'm assuming it's RAM though. I don't know those fancy long names for RAM when you look at the sites to buy them from so I will have a hard time figuring out if it fits my computer.