I still don't like the idea of offering those settings, the energy should rather go into fixing the reasons why the registry keys exist, but here are a few comments:
* The video memory is normally autodetected. You should have something like a checkbox that says "override video memory", or something like 0 = autodetect
* GDI is not the default ddraw renderer. Neither is opengl. It is autodetected based on what the game requests. If you default to GDI, you disable 3D for d3d7 and earlier apps.