Stupid me: Installing the new OpenGL Linux drivers from ATI could not work, since they do not support my older but widespread Radeon7500 Mobility...
...on the other hands, the newest OpenSource XFree Drivers are not that bad, provided one enables 3D Acceleration, which is turned off by default in the configuration files of my Linux distribution!
Now Dom2 runs pretty fine with my Linux-Laptop...
...hence hopefully no more Battery-Problems!
(Although textures are much more coarse, which I understand is due to the different drivers, but also the battle grounds have an entirely different color...much darker and greenisher...more menacing somehow...why?)