Lisa: thank you for your reply. I am aware of the age of the Mac Pro A1186 but I was interested in the video card that would best interact with this old machine. Thanks again for your reply.You do not indicate what you want to upgrade. Is it a video card or perhaps editing apps? You are dealing with a very old system and a very outdated OS version. I suggest you check out Mac Pro | MacRumors Forums
Do some reading. There are some ways to upgrade your system and there will be recommendations for video cards and apps.