I had the same issue with my 4870x2 card. first take off Vert sync if you haven't already, that will limit it to your screen refresh rate, check setting in WoW and graphics card software. Triple Buffer should be unmarked as well (when in game its automatic, but if anything like my Catylist program its seperate from V sync. Since your using Nvidia your going to have to get Riva Tuner to adjust your fan speed and what not cause you might be running a little on the hot side. Lol the other day I took my computer to the local comp shop (I built it myself but was concerned about temps and preformance) well anyways with my side panel off and my video card not getting the extra air from the 2 220 fans on the side it hit 150F degrees and shut it self off during a benchmark test, and that was with my fan setting at 60% needless to say its at 90% now cause its way too loud for me even at 60% lol. and now I have to wait till the heat sync fan set up from Artic Cooling comes out for my card at the end of the month so I can drop another 70 bucks on a 400 card -.-.

Also, if you go from multiboxing to a single instance then you need to make a macro for the following
/console maxfps 0
/console maxfpsbk 0
that will take all limitations on fps off of the current wow.exe in use.

@lowest WoW settings in noobie area/outlands I get 150fps+ (seen sit at 175 as well)
@High Settings Area 52/Nagrand I get 90 to 100 fps
@Ultra Settings Area 52/Nagrand I get 50 to 60 fps

I get the same read outs if i multibox with no FPS limit and just using 1 instance of wow.