I don't play WoW, but EQ2 is CPU-bound and here's a few tricks I used to squeeze more out of raid FPS.
I have *no* idea if any of these would apply to WoW, but with any luck...
1) Turn off, or down combat logging. If you're raiding, this might not be possible since you're probably parsing your DPS. In that case, you could try an SSD, or even a RAM drive for the log. You could also try turning off "others" combat info. Again, if parsing, that might block "others" DPS on your parse window.
2) Turn off, or down, the gfx effects for "others" spells, abilities, etc. With 24 (25 in WoW?) people spamming stuff every second or more, there's a lot of data being sent to your PC to process.
3) Turn off, or turn down other gfx effects - shadows (usually a big one), some types of lighting, etc. Even try lowering the resolution if nothing else is helping, or in combination with the other ideas. Every little bit helps?
4) I've READ that connecting directly to your cable modem can help with some types of lag, instead of one or more switches that connect to the modem. I have NO experience with this since I go thru a switch then router, just repeating what I read on an MMO "lag" tips forum.
5) If the game doesn't support multi-core, try manually setting CPU affinity. Can do this from either Task Manager manually everytime you start the game, or use a custom icon that starts the game on a specific core (I don't remember specifics of the command). Put the game on 1 core, the parser and anything else that takes real CPU time on other cores.
6) Increase the priority of the game process to Above Normal or High. If you're feeling wild and crazy, you can even try Real Time.
That's all I can remember right now, I'll add more later if I can. Good luck!