Try this to see the difference in settings:

1. Go to the video settings and click on the "Defaults" Button. Click OK
2. Press Ctrl-R - you should now see your FPS at bottom of your screen in yellow.
3. Take note of that number.
4. Then run your lo res script.
5. Take note of the number. it should be higher after the script.

As for "tasking your graphics card too much" - the hardware you have will easily run 2 instances of wow (with some tweaking you could probably run 5).

Good luck!