Someone in a german forum (macuser.de, his name is flyproductions) tried several PowerPC-Mac and mac-compatibel-PC-cards. You won't get more than the theoretical 150MB/s of SATA-I since nearly all rely on the firmtek driver which was/is SATA-I. If it is not the firmek driver it is not bootable, but will still give you SATA-I speeds.
Silicone Image Chip 3124, is offered with Mac drivers on the silicon Image website, but I do not remember whether it was bootable. It is at least a cheaper way of adding SATA-ports to your PowerMac than getting a Sonnet one. Sometimes used Firmtek, Acard, SIIG, Macally cards go cheaper on Ebay. LSI, 3ware and Promise also have mac compatible PC-cards, but they are not bootable.
(My Macally bootable card has a VIA chipset, but is not recognised as firmtek - others are -, so maybe there is another driver/firmware in it. It reports itself as SCSI-Device (having SATA and IDE to PCI, not PCIe!), giving the possibility to read SMART data from SATA-drives, but not all drives are recognized. Using an SATA-IDE-adapter on that same card makes the SATA drives recognizeable and SMART readable for whatever reason. Sonnet/Firmtek do support more/every drive.).