When I got my Mac Pro, I was too impatient to wait for the BTO option of a wireless card figuring I'd just get one later. Unfortunately, as other people may have found out, it's not that straightforward to get one as you can't buy it as a customer fit option from most mac stores. After many months of searching and looking for a USB adapter that would work OK, I finally found a mini-pci card that fits internally inside the Mac Pro and works without any additional drivers (it actually uses the same broadcom chipset as the MBA). The card is an HP Wireless WIFI 802.11N 300Mbps Mini PCI-E Card DV TX, and I got one from eBay for £30.00. Fitting it was pretty straightforward if a bit fiddly (particularly attaching the internal aerial wires), and once it was done, my Mac recognised the card immediately (no drivers needed) and it works perfectly. The great thing about this card is it also supports 5Ghz, which many 3rd party cards don't. Hopefully this will be useful to other Mac Pro users out there who have been in the same situation as me.