I have been playing WoW for 7 years now, with multiple different configurations throughout the years and i can tell you this with confidence, upgrading your CPU will *by far) give you the most results.
There is one caveat to this, your GPU does not support DX11, which can give some decent frame boosts alone in WoW, but you also needs windows 7 to enable this in WoW.
Get a new CPU/Mobo/ram (you may also gain large boosts in performance in large areas by upgrading to faster, and 4gb's of memory) and i assume your FPS will double (maybe more).
Down the road find a GTX 460 for your system, you can find these used for 70 bucks now.
I personally am now running a 2500K @ 4.2ghz and a gtx 465 and with everything set to ultra (minus shadows, never use ultra shadows) i get 130 FPS in SW, 60 FPS in crowded areas of SW, and 100-120 fps during raids, only coming down to the 60 FPS mark during intense action.