- Install the latest version of Xcode (you will need this even if your not developing macintosh/iphone applications)
- Install macports for most of your OSS tools (you'll need Xcode first)
- To run tools that aren't OSS or are windows only install virtualbox I know there are alernatives (VM fusion is highly rated) but virtual box is free
- Install an IDE if you do not want to use Xcode as your IDE (for example eclipse and netbeans)
Finally if you have some cash
- Upgrade the ram if required (sw developers love ram especially if you are running a vm :) )
- A second display, I can't imagine developing with less than two displays
