It changes to 800-x600 whilst in fullscreen when selecting F8
Again, how do you know? How do you know it isn't running at a higher resolution, but just looks like 800x600?
It's also doesn't look to be running at a higher resolution, the image is still not crisp
I already said this:
It's a 2D game, resolution forcing won't necessarily look any different than your GPU/monitor scaling.
Resolution forcing is to change the rendering resolution of a 3D game, it's up to the game how to render 2D elements and this game is all 2D elements. You should not use resolution forcing for 2D games.
If you want a fully 2D game to look different, you need to change the scaling algorithm. Either change ImageScaleFactor or ForceNearestPointFilter in the .conf file (or advanced options in dgVoodooCpl.exe).
ForceNearestPointFilter is bad (distortion), but some people who want extreme sharpness might prefer it, it needs ScalingMode set to anything except the first four.
You should set ImageScaleFactor to 0 (this will double 800x600 to 1600x1200).