What is this thing?

Pictures?

 

Like this: http://www.welovemacs.com/9226043.html ?
These are external antennas, one for BT, one for AP. On the back panel (above optical in and out) you'll find sockets for them.

 

You have one of the several G5 towers that were built before the final version in late 2005, when Apple wired the inner casing of the tower with antennas for better airport/bluetooth reception. In other words, your case is not wired, and you are limited to bluetooth 1.0 and airport extreme vis a vis those little external plug in antennas. They do work and most people lose them, so that's good. If you have an airport card plugged in, you will have wireless immediately.