And if you keep going there,
About this Mac > (More Info) > Network > Airport ...
... and see if Airport Hardware, with detailed specifications, shows up. (Consensus seems to be if you do not already know, you probably do not have it since it was optional.)
There was an Amazon seller in the US selling genuine Apple cards for Mac Pro (different ones for different models). They are hard to install because the card, screws, and wires are all tiny.
Now, it seems that users that upgraded or added Airport cards, even Apple cards, had trouble with 10.7 it seems - never would have looked for that as a factor -- and my memory is too foggy to recall why and what the problems that they ran into were.
The first thing as you can tell is the header from System Profiler and "MacPro x,1" lets us and you know what model we are talking about.
How do i know if I have a G4? or a G5?
If it is a Mac Pro it is not a G4 and not a G5.
All Mac Pros have Intel CPUs. The older G3, G4 and G5 machines all had PowerPC chips and were called Power Macs, not Mac Pros.
Mac Pro towers have two optical drive trays (CD/DVD) in front. The PPC PowerMac G5 has only one. Earlier PPC machines like the G3 and the G4 don't have the distinctive aluminum "cheesegrater" housing.
The last G5 was manufactured in the summer of 2006, then Apple switched to Intel in August of that year.
USB WiFi adapters are very cheap and much less troublesome than Airport cards.