Upgrade to Xcode 3

I’m running OSX 10.4 and Xcode 2.5. How do I upgrade to Leopard so I can upgrade to Xcode 3.1?

You’ll have to buy a copy of Leopard. Beware of second-hand copies, too; if they aren’t installation disks or are those supplied with a specific machine, they may not work on yours.

Will Leopard run on a Core Duo mini?

Model: 1.66 GHz Intel Core Duo, 80GB/2GB
AppleScript: none, as yet
Browser: Safari 525.22
Operating System: Mac OS X (10.4)

According to MacTracker it will. MacTracker refers to the model with the specs you list as “Mac mini (Late 2006)” and says that with Firmware Update 1.1 installed it will run the latest version of OS X, which is 10.5.6

Leopard can be installed on every G4 (≥ 867 MHz), G5 or Intel Mac