Per AMD, the HD 4350 supports up to 2560 x 1600 resolution.
http://ati.amd.com/products/Radeonhd4300/specs.html
Does the Hanns-G come with drivers on a CD? The problem is basic Vista / Win 7 applying EDID that comes with the monitor. EDID is roughly "Electronic Device ID" which specifies what the monitor can / cannot do. Apparently the Hanns-G's EDID is incorrect and if monitor driver did not ship with your monitor then getting Vista / Win 7 to ignore EDID is rather iffy.
This is a known issue and MS has released a solution, however, the solution has not worked for everyone apparently. See below link to someone who has a similar issue as you do and it also includes a link to MS's instructions on how to possibly resolve the issue:
http://forums.entechtaiwan.com/index.php?topic=6944.0
Do Google (or Bing) searches on "EDID override" or something along those lines.
Below is a link to a tool that can be used to edit EDID profiles, but I cannot instruct you how to use it and you should do additional research on it before simply using it 'cause it can make the situation even worse.
http://www.tucows.com/preview/329441