|If I were you, I'd get for the Phenom. While kuwese is correct that WoW won't use all 4 cores, many other applications and games will. The Phenom also has an L3 cache, which will give applications sensitive to memory bandwidth a little boost.|
But to answer your question, both CPUs would run WoW at such a high frame rate that you wouldn't be able to notice a difference.
Socket 939 | Dual-core Opteron 185 @ 3.2GHz | 4GB | 1.0TB | Win7 x64
GeForce GTX 460 1GB | X-Fi Ti Pro | A8N32-SLI Deluxe | Antec P182