From a pure quality perspective - and you should know I develop OpenGL graphics software myself - I find ATI/AMD's display drivers to be more consistently of good quality. Both companies flub driver releases occasionally, as they focus primarily on computer game development, but ATI seems to deliver "golden" versions, which just work right with Adobe products, more often. I have had to work around problems in display drivers from nVidia more often.
Also note that Photoshop and company tend to prefer single video cards, rather than dual/multiple GPU arrangements sometimes found inside certain video cards, or creatable by the user by buying multiple cards. Many modern video cards can support multiple monitors on a single GPU.
If you do choose one or the other video card, also keep in mind you'll want to make sure you don't push your computer's power supply past its design limits. I like the ATI x670 line (e.g., the current VisionTek ATI Radeon HD 6670 1GB DDR5) because it balances power consumption against GPU performance, and doesn't emit "on the runway" fan sounds. Don't get me wrong, these cards are plenty fast, but they don't even require a separate power connector.
-Noel
