When you play games does the fps only fall when there is more going on on the screen or is is constantly low cause a lot of games, wow including, have areas where the screen needs to load more. Ie fight scenes with explosions etc in fps games, or in the major cities in wow. They just require more loading.
More then likely, imo it's a GPU limitation. Turn down your settings or get a new GPU.
I have a 4850 n i get around 50-60 fps in high pop areas (in wow) cept of cause dalaran lol. But I used to have an 8800 and it struggled severely with them dropping below 15-20.