Step 1: What you'll need
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.
Step 2: To begin
2. Make sure location is set to "Automatic" and show is set to "Built-in Ethernet."
3. Now where is says "Configure IPv4," select Manually.
4. Enter in 10.0.0.1 as the IP Address.
5. Enter in 255.255.255.0 as your Subnet Mask.
Step 3: Now
Step 4: Back to System Preferences
2. Click on the "Internet" tab.
3. Make sure that on the drop down menu Airport is selected.
4. Click on the box next to "Built-in Ethernet" and hit "Start" at the top.
5. Go ahead and connect your Mac to your XBOX via the ethernet cable.