I imagine that 640x512 overscan is pretty close to 800x600.
Let's see. Overscan of 320x256 was 384x288, so 768x576.
I bet there's a 32 pixel black border on either side, and 24 pixel border on the top and bottom of the display. Not ideal, but otherwise the pixels should be crystal clear (on an 800x600 or 1600x1200 display).
I imagine that the interlace issue is also not a problem now (i.e., no flickering) if the minimig is clever enough to output interlace screens as non-interlace. Can anyone confirm this?
Must get myself one once they're available ready-made...