Low-Res modes?

Back again, this time with a NM2093 (I just can't get rid of those NM's ;-)
I'd like to have low-res modes (512x284, 400x300, 320x240, 320x200) but it 
just doesn't work. 400x300 would be the most important and - if possible - 
stretched over the screen (it's just *2 on a 800x600 screen so it would be a 
very nice mode). However, when I tried to config such modes, they don't 
display correctly. Did anyone manage to get this right? Had the same problem 
with the 2070 so if you managed this on another NM card, please also post it 
here.
The best thing I got up to now was a 400x300 mode stretched to 400x600 on the 
left side of the display. Not very ideal :-(
Any modelines or config hints?

The display is a 800x600 lcd, color depth is 16bpp.