The generated modes in XF86Config
(Modeline ...)
seems always the same, for all videocards.
The problem is that possible dot-clock frequencies
for the videocars chip are different 
for different videocards, so some
high-resolution modes are not available with generated config 
despite the videocard can perfectly support 
such high frequency and resolution.
This requires manual adjustment of "Modeline... " to get high
frequency/resolution modes working.

The improvement which need to be done is the following:
read all possible chip clocks from the videocard
(XFree86 Rel 4 server prints them to stderr)
and then generate modes based on available 
chip clocks. This way the the generated modes
will have the clocks supported by the videocard
and thus the best supported mode can be used by X server.