Thanks for the replies and suggestions.
I'm going to add a 2nd hdd, or more probably, a SSD, to the 1tb hdd that this PowerMac came with from Doffo. I will install MacOSX on the 1tb hdd, and choose a Linux distro to install along side of MorphOS on the SSD, or 2nd hdd. That way I hope to avoid MacOSX messing up my MorphOS and Linux partitions, and I'll just use the Mac firmware early boot selector to choose which OS to boot into. I don't really want to get into some other boot loader, unless it is extremely easy to install and set up.
If you have separate SSD/HDD for MorphOS+Linux, you can use SiI3132 PCI Express SATAII controller.
It don't reach full SATAII speed (300MB/s) but only cca 250MB/s - i.e PCIe-v1 x1 speed, still it is far better then internal disk.
AmigaOS: Micro A1-C, AmigaOne XE, Pegasos II, Sam440ep-flex, AmigaOneX1000
MorphOS: Efika 5200b, Pegasos I, Pegasos II, Powerbook G4, Mac Mini, iMac G5, Powermac G5 Quad