Two questions actually.
1. If I were to buy a Mac Pro with the default graphics card, could I replace it with an nVidia 8800, and actually have it work and all?

2. I see there are options for having multiple graphics cards, do these use SLI to work together? Meaning could I theoretically get 2 8800's and run them wilh SLI?
MacBook Pro Intel Core Duo 2.0ghz, 1gb ram
Dell 228WPF 22" monitor
Mybook Premium 320gb
iPod Nano 4gb (red)
Wacom Intuos3 6x11