Yes. All the slots are x16 size PCI Express, but any PCI Express card will fit (most will be much smaller, and will fit in the right-hand edge of the slot, as you see with the card you have in slot 3.
Just make sure that you use the correct speed slot for your Blackmagic card. You have two x4 and one x16 available. I don't know what speed the Intensity needs.
you have PCIe 2.0
your GT 120 is a rarer single , not double slot wide, graphic card - the ATI 5770 and most are double width
look up specs for your model on site like everymac.com and of course specs and manuals from Apple
www.apple.com/support/macvpro and go to Early 2008 3,1 page
Apple only says that there is 75W per slot and there are two add'l 6-pin power cable plugs for cards that need more than the 75W.
It is rather "hidden in plain site" as far as your hardware specs go.