Just a quick thought, and not sure if it will apply to your vid card or not. I don't know if Ambient Occlusion is a feature supported in the NVidia Driver set for 8600GT's, but if it is, make sure that is turned off. I have a pretty decent system (though, I am using a 9800GX2) and the latest NVidia drivers seem to default to AO set to on, which causes the overall FPS to take a massive hit. Check the NVidia Control Panel under 3d settings, and I believe it's the first item on the list.
For that matter, check all the settings on that list to make sure they are where they need to be. (AA, Triple Buffering if using vsync, etc.) If I am not mistaken, most of those control panel driver settings will tend to override in-game settings, so even if you are telling WOW to run with whatever specifications, your driver settings might be bogging your system down.
I turned off the AO in the Control Panel (IMO that particular eyecandy added relatively little to the overall experience, at least with respect to the performance impact it takes), and damn near doubled my FPS, even in heavily populated areas like Dalaran.
Good luck, hopefully that helps without costing you for new hardware.