If you're running Nvidia cards, you can get the latest version right here. And those with ATI cards can find their new drivers over on their site. Just download the right file for your operating system, run it (you'll probably have to restart your system, so save your work), and then marvel at... OK, it probably won't look too different, but trust me when I tell you you want to be running the newest drivers possible. In fact, if anything will fix those graphical problems people have been seeing in the last patch, this probably will.
But Mike, you say, what if I don't know what kind of card I have? For Windows XP, just right click on My Computer and go to Properties. Choose the Hardware tab, then Device Manager, and then look for the "Display Adapters" menu in the list-- that should tell you the build and brand of your video card. For Mac OS X, you don't have to worry-- Apple updates your drivers for you with Software Update. And for Linux-- well, if you're running WoW on Linux, I'll assume you already know what you're doing, because I sure don't (though you can probably find help here if you need it). And yes, you technically inclined and sharp-eyed ladies and gents, I do call my custom-built, lightning fast XP box "Refridgerator." Why? Because it's freakin' cool, man.