Correct. I don't think it would be much help for CS3 or Aperature. Stay with Geforce 8800GT or the Radeon HD2600 for what you are doing.
The Nvidia Quadro is more for viewing 3D Cad or 3D animation. And actually, for the process of 3D rendering, I believe the GPU isn't important, it's the CPU, and professionals use render farms for this.
Well then if CPU is more important than upgrading to an nvidia 8800gt from an x1900 on the older mac pro, then it would not be necessary to upgrade gpu at all and necessary to upgrade to the much faster 8 cores harpertowns and while at getting a much faster CPU why dont you just upgrade to the new GPU while your at it? Wouldnt you agree?? So in a sense of professional level, it makes sense with what Apple has done.