Hi All,
I just stumbled upon this and thought I would throw in my experience.
I have a Quad Core Mac Pro 1,1 and FCPX 10.0.0 worked fine with my old 1900 graphics card.
I too read about issues on no upgrade allowed for FCPX 10.0.1 for this configuration. So I bought the 5770 and installed. BTW, I am running Snow Leopard 10.6.8 on this machine.
The card installed fine and I just upgraded to 10.0.1 as well. All seems fine there too. I only did the upgrade yesterday, so still in the early stages.
This set up runs a little bit quicker than before. Renders seem faster and thumbnails seem to load faster as well in FCPX.
After Effects renders are way faster than before if this is of any use for anyone.
So, if you are considering the upgrade and want to keep that old Mac 1,1 running. This is a good option.
BTW, I also just picked up a 21″ iMac with the 6770 Raedon Graphics with Lion. And it seems to run FCPX 10.0.1 at about the same as the Mac 1,1 with the new graphics card. This is just to give you a comparison.
Nothing scientific here, just all based on my general sense editing the same project using an external FW800 drive on both machines.