Chances are anything you have/get will be fine. If you start at the commandline with gcc, etc. any used/refurbished machine would do. That would probably even suffice for some basic XCode work. If you move onto 3D, etc. you might need to up the ante. If you are wanting to work on the iPhone you need to be running OS X 10.5, so you'll need a machine that meets its minimum requirements:
A Mac computer with an Intel, PowerPC G5, or Power PC G4 (867 MHz or faster) processor
512 MB memory or more
A DVD drive for installation
9 GB of available disk space or more
In short, there's very little you're likely to want to do for a while that needs the power a pro machine affords.
-Lee