Its EXTREMELY easy to install. It will take you no more than 5 minutes to install it. Basically, you plug in the 2 antenna's hanging loose inside your PowerMac, and then plug the card into the slot and secure it in with the provided plastic clips. The OS will automatically pick up the card with both AirPort and Bluetooth.
The antennas are a little tricky to get snapped into the card. You need to make sure they're straight up and down and you'll need to apply a little bit of pressure to snap them in place. Just try not to snap them in on an angle. Once you figure out how to do the first one, the second one should be fairly easy. As far as I know, it doesn't matter which antenna goes into which hole, unless I just got really lucky when I installed mine.
I just installed this on my 2.3 GHz Dual Core PCIe PowerMac G5 a few weeks ago.