using two videocards

[turbinater]

Try:

X :0.1 | xterm -display :0.1 -e kde3

in one of the main consoles.

I suggest you simply disable the onboard video in the bios(unless your using it for something special).

Now I want to decide at bootpromt or before X starts, which card to use.

I think you can use the "linux single" command at the bootpromt and start X from a console(if you don't mind the hassle).