Need an extra ethernet port

I had been using a wired Linksys router with my Mac Pro. I had a modem, two computers, and an Xbox connected all through Ethernet ports. None of these things have wireless adapters in them. This took up 5 ports on the router.

I just purchased an Airport Extreme, which only has 4 Ethernet ports, so I need to figure out how to add an extra Ethernet port. The Extreme is hooked up to the computer via Ethernet.

From what I understand, these are my options:

1) I can get a wireless card for my Mac Pro, freeing up one of the ports on the AEBS
2) I can get an Ethernet switch and connect it to the AEBS to give me more ports
3) I can get an Airport Express and set it up as WDS(?) and plug one of the things into the Express's Ethernet port

Is this correct, and if so, which one is the best choice? The switch is the cheapest, I know that.