My answer is less related to the GPU and more related to your game setup.
Do you have any addons, if so do you have any addons that scan an area. (haven't played for a while, but for example some scan for the opposite faction when in certain places, which can cause tremendous frame drops)
If you are running addons, just trying turning EVERYTHING off.
That's my say on the game related side.
Whether it's the GPU's drivers or a genuine bottleneck, I have no idea, I'll leave someone else to be the judge of that.
Edit: Theoretically that card should be enough to smash wow @ fps right? My 5770 manages Ultra (1080p res) and I get the similar FPS. (Ok yes I know lower resolution, but come on.. We're talking about low end vs enthusiast end.)
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