Only 27-inch iMacs and iMacs with Thunderbolt ports can be used as an external display > http://support.apple.com/kb/PH11302?viewlocale=en_US&locale=en_US
After setting up your Mac Pro, you will be able to use the iMac as an external display for the Mac Pro with an application like ScreenRecycler, but you need another monitor to set it up. In your case, you have to buy an external display to use the Mac Pro
Mende is right about the iMac as a display.
There is one more obscure feature to consider:
If the Mac Pro has a display card in it (it needs a place to build the screen image) you could use the Ethernet connections (you already have in place for Internet access) to do Screen Sharing. This would allow you to have casual access to the Mac Pro, while continuing to use the iMac as a computer.
I deliberately say Ethernet because some users have found that Wireless connection is not quite fast enough to do this in a satisfactory way. But wireless would let you try out the feature and see if it will work for you.
You would probably have to borrow a display from a friend or work with the genius Bar folks to get it set up initially.Mac Pro (Early 2009), Mac OS X (10.6.8), & Server, PPC, & AppleTalk Printers