I run two labs with 2008 Mac Pros. To keep things humming along with FCPX, I added 16 GB of RAM and an additional 2 GB Nvidia Geforce GT640 card to complement the stock 512 MB card in each machine. Everything worked great up through Mavericks. Then when I updated to Yosemite, the Mac Pros went into a boot loop. The machines were unusable. I read that there was a problem with systems that had more than one video card installed. So I took out the original video card. Interestingly, about half the Mac Pros ran fine and the other half stayed in a boot loop. So I removed the new Nvidia cards, keeping the stock ones instead, upgraded to El Capitan and now things are running fine. FCPX isn't running as fast, but at least the computers are stable. I have no idea what Apple did with its updates since Mavericks to cause the computers to freak out with non-Apple video cards. But that sort of defeats the whole point of having a machine with lots of expandability. Has anyone found sizable video cards that do work with those old Mac Pros and El Capitan?