> 2) Grfconfig installs on startup the right video modes but...
> when i start 'startx' i have some error messages and then the system hangs.

 We need those error messages to help you.

> If i config X with "-dev /dev/grf0 -depth 680 -height 540 -depth
> 4 -emulatemiddle" the error is: 'Warning: cannot allocate colormap entry for
> "color"' and 'twm: unable to allocate color "gray70"', then hangs...

 What does make you think "it hangs"? Ignore this error message for
instance, it is not fatal. 

 So far your setup looks fine to me. X11 starts fine and healthy, even
with twm. Now, what is bothering you?

-- 
Markus Illenseer