How to config X

[Markus]

That's weird. With all modelines commented out I still get:

markus@mrk-fujibox:~$ xrandr
 SZ:    Pixels          Physical       Refresh
*0   1280 x 1024   ( 342mm x 271mm )  *60
 1   1280 x 960    ( 342mm x 271mm )   60
 2   1152 x 864    ( 342mm x 271mm )   75
 3   1152 x 768    ( 342mm x 271mm )   55
 4   1024 x 768    ( 342mm x 271mm )   75   70   60
 5    832 x 624    ( 342mm x 271mm )   75
 6    800 x 600    ( 342mm x 271mm )   75   72   60   56
 7    640 x 480    ( 342mm x 271mm )   75   73   60
Current rotation - normal
Current reflection - none
Rotations possible - normal
Reflections possible - none

So, try screen=1024x768 instead of fb.
Did you try running xf86cfg and it's expert settings?