Short of upgrading there are some things you can try.

If you have multiple monitors, I'd highly recommend dropping SLI. You may only be using one of your video cards anyway. With ISBoxer 38 there are new Window Layout presets in the wizard for multiple monitor/video card setups. http://isboxer.com/index.php/guides/...le-video-cards

Drop the background FPS setting in ISBoxer, for each of your slots, to 15 or so (default is 30). Keeping the background slots at 10-15 shouldn't hurt anything for WoW and this will free up a bit of both CPU and GPU.

And you can also drop the video settings on your slaves to minimum at that point, if you haven't.