installing Jaunty i cannot get a higher resolution then 800x600.

Display is: Medion MD 9760AI Type M150P1
Expected resuolution: 1024x768 at 60 Hz

As display is not recognized i manually changed xconf.org by using modeline as follows:
Section "Monitor"
Identifier "Configured Monitor"
Modeline "1024x768_60.00" 64.11 1024 1080 1184 1344 768 769 772 795 -HSync +Vsync

Section "Screen"
Identifier "Default Screen"
Monitor "Configured Monitor"
Device "Configured Video Device"
DefaultDepth 24
SubSection "Display"
Depth 24
Modes "1024x768_60.00"

but nothing happend nor the resolution changed or is possible now to change.
Anybody any help idea what to do?
Help would be greatly appreciated