> Here is where late 2005 G5 support would be nice.
> We could at least upgrade to an SATA2 controller.

The 133 MHz PCI-X slot of older G5s is enough to not limit one SATA3 port even. While SATA3 cards may not exist for PCI-X, SATA2 cards definitely do, theoretically being able to use up to 3 SATA2 ports simultaneously.

> An SATA3 driver might also be possible, depending on
> the transfer limitations of PCIe Gen1.

3 PCIe v1 lanes are enough to not limit one SATA3 port. There are SATA3 cards for up to 8 PCIe v1 lanes, which "late 2005 G5" happens to have a slot for, resulting in the theoretical ability to use up to 3 SATA3 ports simultaneously.