Multi-Core Cpu Architecture and WoW / Lag and Graphical tweaks...Guide
The TLDR version of this guide
Also, as a little side note. If you decide to edit your config file you obviously need to open it. Right click on it, click open with, and open it in notepad, word, or any other text editor.
Updated for 4.0:
They changed wow in 3.1 to run on more processor threads. Originally it was running on 2 main threads with several smaller threads handling simpler tasks. As of 3.1 there are now 3 main threads, and many more smaller threads. This means that forcing wow to run over all 3 cores may prove more beneficial then having it run over only 2 cores, as now you can"force" it to use all of the available cores.