I haven't played World of Warcraft in a while, but if you can run Oblivion and Fallout 3 fine, it is not your machine. My laptop couldn't run either of those even on super low settings and it did WoW fine. So, I would imagine it is your internet. This might have changed since last I played, but if you look to the bottom right corner of your screen you should see what looks like a computer, its screen will either be green, yellow, or red depending on your latency. If you mouse over it, it will tell you your current frames-per-second and your latency. If your latency is high that either means your server is having problems, your internet is not fast enough, or both.
You also might want to look around the World of Warcraft support page. http://us.blizzard.com/support/index.xml?locale=en_US&gameId=11 If you cannot solve your problem, you might want to contact their customer service via e-mail or phone. I've had to do it a few times. They've always been really nice and helpful for me.