Thunderbolt needs a Thunderbolt port on the Mac. An older Mac with Mini Display port could be connected to a Cinema Display.
I think it is time to start planning the retirement of the 2008 Mac Pro. There is too much it is missing....USB 3, Thunderbolt 1 & 2, 4K capabilities. Donate it to Good Will for a tax credit.
Even consider a new iMac if we get an updated one announced in the fall with Retina display, TB2, DDR4 memory, and latest Intel CPU and chipset.