That system should be able to run WOW without any problems. I would suggest you check not only your FPS but your Network latency. While in WOW, mouse over the Game menu icon but dont click on it. Check the latency values that appear on the popup. If they are listed in red then that is the problem.
Also, try setting your graphics options in WOW to Good. This is the medium graphics setting.
High latency is usually caused by a problem with your internet connection. It can also cause jumpy game play and intermittent disconnects. Are you having any problems with disconnects? Which type of internet connection are you using and what speed is it, DSL, cable, Wireless(using a dish on your house)? Most DSL and Cable internet systems are plenty fast for WOW but Wireless often is not.