Hi DerJochen,
I have the same Notebook too - Asus G73 JH / 5870mobility
I think it was only a driverproblem at your notebook.
My G73 runs with medium textures at 1920x1200 between 29 - 130 FPS
Now my questions to you:
1. Do you have fixed the G73 grafix-bios problems?
2. Which ATI/AMD Catalyst do you try?
PS. greetings from germany