The problem with running Yose or El Cap on a 1,1 Mac Pro is not only that the stock video card isn't compatible, but also that the machine is not on the 'installable machines'. The central point of contention is that the Mac Pro 1,1 has an interesting motherboard.
Buy a compatible graphics card. Buy an external HD enclosure.
Take out your boot disk and put it in the enclosure. Plug it into your second computer with the installer downloaded. It's documented all over the place.
Boot your second computer and install OS X onto the disk in the enclosure. Reboot and look for all the relevant plists to insert your platform ID into on the enclosure hard disk so the computer is fooled. This is necessary to accommodate the larger fan on your new graphics card.
You may now remove your current graphics card by gently rocking it back and forth while pulling it towards you out of the Mac Pro. Refer back to the black clips mentioned earlier Figure 2.
If your GPU came with power cables this is where you will connect them some smaller cards do not require and thus do no come with power cables. Insert the small black end of each power cable into the clip until they "snap" into place. Before inserting your new graphics card take note of the bottom-most PCIe slot D.
Just like your old graphics card, your new graphics card has a connector tab along its edge that will fit in this slot. Make sure to line up the connector tab and the PCIe slot correctly and apply gentle pressure until the card "snaps" into place.
This should not involve excessive force. We believe in the long term value of Apple hardware. You should be able to use your Apple gear as long as it helps you remain productive and meets your needs, upgrading only as necessary.
We want to help maximize the life of your Apple gear. In contrast to the age of the older first Mac Pros, there is still some new hardware that is being released, which can be used on these Mac Pros, primarily due to the software support available. The idea behind this was being able to create a very powerful, no-questions-asked workhorse — and it worked out just fine. This is not a perfect solution, but it can work perfectly when setup and generally not messed around with.
I picked up 2x 6-to-8 pin PCIe adapters for the card. Even under full load, this is still within the designed power capacity of the power rails. I mostly play the game Arizona Sunshine on the full resolution, with medium textures and shadows, and always hit the 90fps mark.