After updating to 10.3.9 and using h24 things worked quite well. All crashing was gone. The only problem I encountered was a delay of up to almost 2 seconds on video in if I was running any kind of media on the other display.

So my solution ended up changing the G5 to another machine that has faster drives, 4 gigs of RAM, Os 10.4.6 and an x800 graphics card.

But now everything works!

BTW, does the graphics card have any significant influence on Catalyst performance?