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.).
I guess i will install the ssd in a 1.5 Gb/s drive bay. But I read on some forums that there are problems with ssds such as OCZ Vertex. Can anyone confirm that at least the OWC 3 GB/s are backward compatible with the slower bus from the PowerMac G5? And do they boot significantly faster?
I use OWC's SSD drives in 3 PPC systems, 2 G5 towers and a G4 Cube.
In each case the increase in speed, both booting and App loading is very noticeable and IMHO well worth the installation, especially now that the SSD prices have moderated quite a bit. I did my first installs when the SSD's were considered expensive, but still never regretted the decision for a moment.
I recommend any SSD that uses the SandForce controller, such as the OWC or OCZ Vertex 2 drives. You want to avoid SSDs that use SATA III as the G5s are a bit iffy with these drives. However, today, it is hard to find SATA II based drives. Newegg carries refurbished OCZ Vertex 2s for pretty cheap, or you can get an OWC Electra 3G for a fair price. I use the OWCs in my PowerMacs and the speed is great!
Funny thing is Sonnet advertises PCIe Cards with SATA-II (and III) on their website, but from a test another user did, you still seem to get only SATA-I speeds on a G5. If you get the card I would like to hear your experience to double check what the other user in another forum said (if the SSD goes over 150MB/s is the interesting thing. Please don't use benchmark utilities when testing, just copy/duplicate a file of 20GB and stop the time and then calculate what MB/s that gives. I no longer trust benchmark utilities.)