Thank you for this hint. It was a little short in description, but google was my friend :-)
The device ID of my graphics board is nVidia Corporation G98 [GeForce 8400 GS] [10de:06e4] (rev a1).
I got that from the system information of my Lion installation.
So I patched info.plist of /System/Library/Extensions/NVDANV50Hal.kext and
added 0x06e410de&0xffe0ffff to <key>IOPCIPrimaryMatch</key>.
After the reboot the full resolution of the monitor (1920 x 1200) was achieved.