What's a good graphics card for Yosemite on a Mac Pro 1,1?
I ended up going with the aforementioned card (the "XFX HD 6670 650MHz 2GB DDR3/HDMI/DVI/VGA PCI-Express Graphics Card HD667XCLF3") and it worked brilliantly in both Lion and Yosemite.

I've only been using the DVI output, not HDMI, so I can't say anything for sure about the HDMI output.