Result: 2 MB VIDEOMEMORY should be enough when using a resolution of 800x600. Since we all love screendragging, we need 2 MB extra VIDEOMEMORY, so after all 4MB VIDEOMEMORY should be used.
Remember, though, that we don't have 24- or 32-bit support with Minimig RTG - so in practice 3 meg is fine for 800x600x16.