What's the PCI Compatibility for Mac Pro, Early 2008?
The everymac.com website reports the following (which basically is the same as Apple reports:


  This model has two PCIe x4 expansion slots and two PCIe 2.0 x16 expansion slots (in the default confugration one PCIe 2.0 x16 slot is occupied by the graphics card). Apple also reports that "all slots provide mechanical support for 16-lane cards [and supports] 300W combined maximum for all PCI Express slots."


Note: The two PCIe x4 expansion slots are actually PCIe 1.1 x4 expansion slots. PCIe 1.1 has the same data rate as PCIe 1.0.

Wikipedia offers a nice table which is repeated below.



Below is a quote from Wikipedia regarding USB 3.0.


  USB 3.0 adds the new transfer rate referred to as SuperSpeed USB (SS) that can transfer data at up to 5 Gbit/s (625 MB/s), which is about 10 times faster than the USB 2.0 standard. 


Wikipedia also has the table shown below.


Almost everything made for PCIe and USB is as backward compatible as reasonably as possible. Which means some cards may work in your Mac, but not necessary at at a high enough speed to get the performance advantages of USB 3.

This OWC website offers a Allegro USB 3.0 4-Port card for $59 (USD) and explicitly states the card is for your model Mac. However, how the card preforms depends on the slot you use. Under "Bus Interface", the specifications state "PCI Express 2.0 x1", where "PCI Express means "PCIe" and "x1" means the card only has a single lane for data. According to the "PCI Express link performance" table, if you put the card in the PCIe 2.0 slot, then the transfer between the card and the Mac will occurs at 500 MB/s. According to the "USB 3.2 transfer modes" table USB 3.1 Gen 1 (or USB 3.0) transfers data a 625 MB/s. So the actual transfer rate to the Mac will be the lower of the two which is the PCIe speed of 500 MB/s. If you use one of the PCIe slots in your Mac, then the transfer rate will be only 250 MB/s.

This OWC website offers a Allegro USB-C 4-Port card for $149 (USD), which is also for your model Mac. Again, how the card preforms depends on the slot you use. Under "Bus Interface", this time the specifications state "PCI Express 2.0 x4", which means this card has a four lanes for data. According to the "PCI Express link performance" table, if you put the card in the PCIe 2.0 slot, then the transfer between the card and the Mac will occurs at 2 GB/s. According to the "USB 3.2 transfer modes" table USB 3.1 Gen 2 transfers data a 1.25 MB/s. So the actual transfer rate to the Mac will be the lower of the two which is USB speed of 1.25 MB/s. If you use one of the PCIe slots in your Mac, then the transfer rate will be only 1 GB/s. While this would not slow down a USB 3.1 Gen 1 (or USB 3.0) device, it would limit a USB 3.1 Gen 2 device from 1.25 GB/s to 1 GB/s.