The PCI-X card 64bit gets 62MB/s max in my PowerMac G4 (real speed maesured, not with a benchmark tool, they will give you 80MB/s and more).
I get stuttering from time to time with this, when watching a part in iMovie or a DV file in Quicktime. I am not sure, if it is the driver or the poor FSB of the G4 of 100MHz. I guess MDD G4s are better for these cards, since Sonnet sold PCI-X cards for the MDD.
I keep the card in my AGP G4, though, because of the transfer speed.
The PCI 32bit only gives me around 35MB/s in this G4.
Regarding the card with the Chip "PowerPC" on it. I saw this on LSI 3ware 9650SE (and some other) cards. Is has nothing to do with bootability on Macs, but this manufacturer has drivers for these cards, that allow you to use the card for Drives, as long as you do not boot from them.
The whole PCI capabilities of the PowerMacs can be seen at everymac.com