Well, IMO your bottlenecks are all so close to each other that you really aren't going to buy much by upgrading anything individually. WoW is less CPU-bound than it used to be but it is still CPU-intensive enough that increasing resolution or FPS will push your CPU use up as well, resulting in marginal gains in playability. Do you use slot-swap detail macros e.g. stuff that adjusts renderscale, etc.?