I know it isn't 1080p, because I sometimes drop below 30 in LFR or while raiding 25 man. Probably something like 768.
yeah i run wow at 1080p with a gtx670 and i sometimes get sub 40 fps drops in lfr (having said that im using a fx4100 buldozer so it bottlenecks my whole rig)
1400*900 is about half as many pixels to draw as 1920*1080
which is why i chose a 1400x900 monitor to get the best visual performance out of my hardware. if i went with a I5 and a 79xx, or a gtx 670 and above i would have chose a 1920x1080 monitor.