Well RAM is only a third of your problem probably.
1. is that you play WoW, go play FFXI
2. you probably need to check your video card
3. RAM
No doubt you are running Windows XP and running just fine on that does require 512 megs of RAM to run. So upgrading to 1024 mb definitely good idea to get smoother loads through programs.
Though playing WoW you will need a good video card as much as RAM, because RAM will only take you so far until you need must upgrade the video card. Now if you have a great one already, then yes it is the RAM, if not, it is either the video card or the both. I would say get the card because you will need a better one eventually. And gig of RAM should do you well for a while as well.