1.) Thanks to all.

2.) The solution of my problems:

My main problem was starting the Xserver. I could make the console run using options like nodcc, xvrefresh and screen.
Within the Xserver, where I saw "Mode not supported" again, I tried [ALT] + [CTRL] + '+' / '-'. This worked, but the resolution was about 600x800 on a 18" tft screen. Within the configuration manager I saw two things:
Maybe the monitor was detected wrong (or it is the same one with different name!?). But the card was not detected correctly. I got a Radeon 7000. Detected was VESA. This combination didn't work and so I used xmodul=radeaon and even xserver works for me...