I know you are looking for some specific tips on how to make World of Warcraft run faster in Vista. I could a smart Alec and tell you to get Windows 7, but that's probably not what you want to hear. In fact, I'm going to assume that you do not want to have to do any hardware upgrades either, so that cuts out suggestions for getting additional memory or a new video card. Just keep in mind, that if your performance is really bad, then you may just wind up doing that in the future. You'll always get your best performance increases by upgrading your hardware.
But since you are not in the market to do that yet, let's see what we can do about getting a bit better performance from your current computer with Vista. Follow these tips and you should be able to eek out a bit more framerate while playing WoW.
1. Make sure you video card drivers are up to date
This is pretty self-explanatory. You can find the make and model of your graphics card in the Device Manager. Updated drivers are generally available at the manufacturer's website.
2. Run WoW in full-screen mode rather than windowed
If you normally run World of Warcraft in a window, switching to full-screen mode should give you a dramatic increase in FPS.
3. Turn off Vista Special Effects
You can do set WoW up to turn off a few of Vista's special features and effects each time you play. In the "compatibility" tab of the Property menu (right-click your WoW icon), disable the following: Desktop composition, visual themes and "display scaling on high DP settings".
4. Turn off the Windows Sidebar
I hate to turn this feature off because I find it quite handy with the various gadgets, but it can be a resource hog, so it is probably best to dump it. You can right-click on the sidebar and select "Properties" to change the settings for startup. Alternately, you can always just close it when you are ready to play World of Warcraft.
5. Use Readyboost
Okay, this is the only "hardware upgrade" tip I'm going to give you because it is easy and cheap. Get a USB flash drive and take advantage of Readyboost. When you insert the flash drive, just select to use it for ReadyBoost (Speed Up My System). Ideally use a flash drive the same size as your computer's physical memory.