I just installed a bluetooth card and Airport Extreme card in my first generation Mac Pro. It’s actually fairly simple once you know where to look (using this article as a basic guide). What I did have trouble with is finding out what size of mounting screw was required. I ordered both cards from DV Warehouse but I guess I failed to inquire about mounting hardware.
An exhaustive search online revealed nothing about the thread size so I took the smallest machine screw I had (about an M2.5) and wondered down to Huron Fasteners and told them I needed something smaller. They gave me a sample M2 to take home and try and sure enough that was the right size.
So for anyone else looking for this information, a first generation Mac Pro requires “M2-0.4 x 4” mounting screws to install a wifi and bluetooth card. Hope this helps.
To read more about the actual installation, read this article — How To: Installing an Airport Extreme Card in an Intel Mac Pro « Meandering Passage.