I have heard that the scandoublers don't work with 800x600 as it's not one of the PAL modes. If your monitor supports it you may use it in interlace, but it seems like many monitors doesn't support it.
You may find screen drivers on the Aminet that gives you horizontal resolutions close to 800 pixels, about 770 if I remember right (using overscan). This should possible be the best solution for internet as 640 pixels are not enough.
Have you tried 1280x512 interlaced through the scandoubler? Haven't myself as I don't have a scandoubler, could be nice in case it works.