The nVidia cards have better drivers for multi monitor gaming support, but I think the ATI ones will allow for what you want to do. With nVidia's cards you can span across monitors and should be able to have different views as well. Keep in mind the video card now has to drive two monitors, so get as fast a card as you can, preferably a GeForce 5950, or if you can afford it one of the new GeForce 6800s.
The ATI cards don't allow views to be split across the monitors, but will allow you to have different views or windows open on teh second screen. ATI doesn't seem to save the configuration either, so you need to move the windows around each time you start the program.
A PCI card will allow you to have multiple windows open, but won't allow for having one view spread across both monitors.
Not all games support this either outside of flight sims.
Have fun! 
goates