Trying to upgrade to Sierra, I need to upgrade WIFI card.

I have a Mac Pro 3.1 and as you have found the internal Airport (WiFi) module is not compatible with Sierra, so I use an Edimax EW-7833UAC USB dongle which works well.

I am running Sierra successfully on a 2007 24" iMac (7,1) after upgrading the CPU from a T7700 to a T9500 (Penryn) and applying dosdude's Sierra patch. (If your 2008 already has a Penryn chip you don't need to do that part!) Also having the Broadcom wifi incompatibility problem, but I don't really need that on this machine. The unsupported wifi does not prevent Sierra from running. The system just doesn't see any wifi hardware. If I do someday want to replace the wifi hardware, does anyone know what model would work and has the right form factor for this machine?

Sierra will run on that machine, just the standard wifi card will not work, you can replace the standard Broadcom card with a later card, or buy the upgrade that is sold on Amazon. If you do not want to start taking your Mac Pro apart have a search for a suitable wifi dongle, which is what I did.

Easy, just buy edimax N150 wifi nano USB adapter, install the dmg driver, restart than you should see the wireless utility icon on the top right. I tested it on a imac 2007 upgraded to high sierra, with dosdude1 patch. Working flawlessly.

"I think the obvious point should be made that you can simply use a wired Ethernet connection and forego replacing the wIfI module.

I live in Ontario and my ISP (Bell Canada) gives out WiFi Pods that have an Ethernet port on them, I’ve been using my WifI thru the wired Ethernet like this for months because of the higher 5G (AC) performance."