If you look at the Apple Store page for buying a Mac Pro, you'll see it comes with the ATI Radeon HD 5770 1GB.
Under "Second Display," it says:
All Mac Pro graphics cards support up to three displays: two using Mini DisplayPort and one using DVI.
Under "Graphics," it says:
Configure your Mac Pro with two cards to power up to six displays simultaneously for visualization projects and large display walls.
If you click on the "Learn more" link under Graphics, it says:
ATI Radeon HD 5770
Ideal for motion graphics, 3D modeling, rendering, or animation, the ATI Radeon HD 5770 with 1GB of fast GDDR5 memory comes standard — and provides up to 5x more performance than previous Mac Pro standard graphics cards. It also includes two Mini DisplayPort outputs and a dual-link DVI port.
Dual-display and triple-display modes
Both ATI Radeon HD cards each have two Mini DisplayPort outputs and one dual-link DVI output for a total of three video ports. All three ports are available for simultaneous use, depending upon the type of connection…
Note: If you are using more than one Mini DisplayPort to Single-Link DVI adapter, only two displays are supported across the three ports. To connect up to two Mini DisplayPort displays and up to a 30-inch DVI display simultaneously, use the ports without any adapters. To connect two DVI displays, use the dual-link DVI port and the Apple Mini DisplayPort to DVI Adapter or the Apple Mini DisplayPort to Dual-Link DVI Adapter (sold separately). To connect three DVI displays at once, you must use two Apple Mini DisplayPort to Dual-Link DVI Adapters (sold separately). To connect up to three VGA displays simultaneously, use the Mini DisplayPort to VGA adapter and DVI to VGA adapters (sold separately).
However, based on this blog post, it's worth reading these two Knowledge Base articles before making a buying decision: