Thoes older Dual G5's can run anything you throw at them... as long as you ad some RAM to the Base Configuration. 2 - 4 GB would be enough for doing 3D & Video work. The graphics Cards were Fine for just about anything.
I work at a University, and we had 6 of the Dual 2 GHZ, which still RUN great. We just got 3 new 2Ghz Dual Core models and 3 Quad core Models through a Grant. They are are fine for now except that we need to upgrade the RAM badly. (They only came with 512 MB) 😱 😱
I dont know how much of a benefit PCI-E is over the older models. sorry.
I dont think the G5's dont have AGP card slots so you cant use an Nvdia 6800 AGP card.