The card needs drivers. A card based on standards that the OS already supports should work. USB OHCI or EHCI. FireWire OHCI. You may be able to use PCIe GPUs that work in a Quad G5 in an Old World Mac running Mac OS X 10.4.11. Nvidia 6600 or 7800 or similar cards require a
modified firmware for Old World Macs. SATA AHCI may work in 10.5. Like I said before, it doesn't need to be PCIe. You can connect Mac PCI cards to PCIe and have the PCIe connected to Mac PCI.
There are many options to choose from. I have not tested most of these options. The ones I tried I have not finished testing (which means I don't know if they work completely and I haven't gathered performance results yet).
PCI to PCIe:
https://www.startech.com/en-ca/cards-adapters/pci1pex1
PCIe to PCI:
https://www.startech.com/en-ca/cards-adapters/pex1pci1
You may want to use some standoffs to secure the PCI back plate of whatever you connect on top of these to the PCI slot openings of your computer case.
PCIe to two PCI:
https://www.amazon.ca/dp/B00KZHDSLQ
PCIe x1 to four PCIe x1: These vary by where the bridge is located and the width of the slots. Usually a computer case will have PCI slot openings with a spacing of 0.8 inches or 20.32mm.
https://www.amazon.ca/dp/B08CL16MZ8
https://www.amazon.ca/dp/B092R64W2V
https://www.amazon.ca/dp/B09WD7HXR1 This one has six connections which may be overkill - this page has links to alternate four and five and six downstream connections options.
https://www.amazon.ca/dp/B097H4BJR8 I don't know if these slots are 20.32 mm. I had trouble with one of these that had 3 slots. It would detect directly connected USB controllers but not NVMe controllers. They may have power issues and you may be expected to use separate PCIe x1 to x4 or x16 risers for each x1 slot. This one comes with standoffs - hopefully they are the right length if you want to place this in an existing computer case that has available PCI slot openings.
PCIe x1 to PCIe x1 (x4 physical):
https://www.amazon.ca/dp/B09HTFDNFW These might fit under a two slot GPU. These look narrower than 20.32 mm so they might be good for using existing PCI slot openings. Hopefully the 3.3V regulator takes power from 12V or 5V and you can just use a molex to SATA power adapter. These are two small to have room for standoffs so maybe some rubber feet can used to raise the rise enough for existing PCI slot openings.