I saw another guide on how to do this on here but it was horribly wrong and left out so much stuff, so I decided to make my own.
Step 1: What you'll need
You don't really need much to do this.
All you need is:
A Mac (this guide is for 10.4 but im sure it would be very similar on 10.5)
And XBOX 360 (duh)
A wireless network to connect your Mac to (your own)
The ethernet cable that came with the XBOX
AT THIS POINT LOOK UP HOW TO SET A STATIC IP ADDRESS AND DO IT.