Airport Extreme card for an Early 2008 Mac Pro

I haven't seen any "official" side-by-side tests, but anything is possible. What's more critical is antenna design over that of the computer's wireless interface. The number of spatial streams that can be processed will not be affected by that interface, but does affect the overall theoretical throughput that the wireless device can work with.

Either way, you should review the vendor's return policy BEFORE you purchase, especially if you end up not being satisfied with the product.

The alternative to USB is to buy a wireless bridge..

An airport express can do this but if you get an idea of the brand of wifi router you are connecting to .. a closer match might be easier.. and cheaper. And it can work as repeater and bridge at the same time which the express cannot do.

Overall this is IMHO easier because it is just plugged into the Mac by ethernet. No drivers and no issues.

The speeds also tend to be better because you can locate the wireless bridge in a better location and just run longer ethernet.

On a MacPro you do have lots of choice though.. there are TP-Link wireless cards that work natively on Mac OS and they are cheap and readily available.. you will need to look up suitable ones.

TL-WDN4800 is one well known to work.. it is PCI Express and I am not sure of the slots in a 2008 MacPro but you should be able to find a plain PCI if that is all you have.

You can also buy adapter cards for PCI or PCI Express to various Mac Laptop cards which are readily available. This means you have true Mac card but it will be a lot newer and a lot more capable than an original wireless card from 2008 era.

Note that a card inside a computer will have antenna in exactly the wrong place for good wireless reception and why IMHO a wireless repeater/bridge is actually a better and often cheaper alternative.

Instead of an internal Wi-Fi card, you may want to consider using a wireless USB adapter instead. One outfit that sells these is Other World Computing (OWC).