The reverse is also true. An Nvidia card will work in an ATI chipset also.
The only time you'll run into problems is if your trying to use two video cards. (SLI/Crossfire) If you want to run SLI, you need 2 Nvidia cards, with an Nvidia chipset. Many people will buy an Nvidia card with an Nvidia chipset just incase they feel the need to put a second card in.