Macintosh bootable volumes are supported on the internal 2-port PCI card (part number TSATA) but are not supported on any of the other SATA cards at this time.
Some customers have experienced problems with the current version of the firmware in older G4 machines. Sonnet suggests downgrading the firmware to the Version 5.13, which can be found at http://www.firmtek.com/download/SeriTek1S2_513.zip