I plan on giving my 2007 Mac Pro to someone. It's possible that she will need to use WiFi rather than connect directly via Ethernet cable. Unfortunately, the Mac Pro does not have the Airport Extreme card that it needs.
Some models of mac pro shipped with airport
capability but didn't have it. In other words they had the antennas and the connector on the mobo but didn't come with the card installed. For those models, you could purchase an "upgrade kit" from Apple. I got to install a few of them, but it was somewhat haphazard. Each model had a different kit, and they weren't interchangeable. Although the cards appeared identical, they used slightly different firmware specific to that model of mobo. (I missed the powermac g5's that could be upgraded with a standard G card...)
So the first check was to make sure it had the antennas installed. They were hidden, but you could spot the dangling end cables often tucked behind the mobo in the upper left corner, near a mini PCI sloth that had two screw hole posts near it where the card mounts. If you have that, you have to track down the correct card, and a pair of screws.
Considering the age, you're not going to get it directly from Apple. You'll have to go some other route, or perhaps salvage one from another machine. Finding those kits, even when they were supported, was surprisingly difficult. Good luck!