Heureka! 
So the issue was software drivers: following the instructions at http://www.reddit.com/r/litecoinmining/comments/1c4n7i/guide_getting_started_with_n_x_radeon_7950_on/ and updating my AMD drivers (Step #8) did the trick .. but now I've another problem: Overheating 
Hot GPU:
Cold GPU:
Although the upper RPM is crazy the heat conduction does not seem to be as great as the lower GPU; I opened my case to increase air circulation but does not seem to help that much .. I think I'll replace my GPU 0 which is a fat ASUS with a thinner Sapphire to have more space for air circulation, or just rip everything out of the case and put them into an open laundry basket or something like that.
So the issue was software drivers: following the instructions at http://www.reddit.com/r/litecoinmining/comments/1c4n7i/guide_getting_started_with_n_x_radeon_7950_on/ and updating my AMD drivers (Step #8) did the trick .. but now I've another problem: Overheating Hot GPU:
Code:
GPU 0: 593.7 / 466.6 Kh/s | A:14 R:0 HW:0 U:2.61/m I:20
87.0 C F: 85% (3592 RPM) E: 1100 MHz M: 1500 Mhz V: 1.090V A: 99% P: 20%
Last initialised: [2013-05-02 13:58:33]
Intensity: 20
Thread 0: 597.5 Kh/s Enabled ALIVE
87.0 C F: 85% (3592 RPM) E: 1100 MHz M: 1500 Mhz V: 1.090V A: 99% P: 20%
Last initialised: [2013-05-02 13:58:33]
Intensity: 20
Thread 0: 597.5 Kh/s Enabled ALIVE
Cold GPU:
Code:
GPU 1: 596.8 / 590.7 Kh/s | A:27 R:0 HW:0 U:5.04/m I:20
72.0 C F: 37% (2138 RPM) E: 1100 MHz M: 1500 Mhz V: 1.090V A: 99% P: 20%
Last initialised: [2013-05-02 13:58:33]
Intensity: 20
Thread 1: 595.9 Kh/s Enabled ALIVE
72.0 C F: 37% (2138 RPM) E: 1100 MHz M: 1500 Mhz V: 1.090V A: 99% P: 20%
Last initialised: [2013-05-02 13:58:33]
Intensity: 20
Thread 1: 595.9 Kh/s Enabled ALIVE
Although the upper RPM is crazy the heat conduction does not seem to be as great as the lower GPU; I opened my case to increase air circulation but does not seem to help that much .. I think I'll replace my GPU 0 which is a fat ASUS with a thinner Sapphire to have more space for air circulation, or just rip everything out of the case and put them into an open laundry basket or something like that.