I noticed that compiz wasn't activated in xfce4 so I ran it from a terminal and now I get all kinds of effects lightening fast. I also get real video playback and little Xorg CPU consumption.
Thanks a great deal to all that helped me here !!!
This brings back some faith in ATI cards. I just hope that they keep on supporting open source driver development for their high end cards (and improve the performance of the hardware of course ;-) )
I'll be building another machine for somebody else with a 8500 GT card. From this I'll get some more experience with the NVIDIA so I can judge better if ATI or NVIDIA is the way to go. I'll report back!