Okay, so I got a Sil 3114 based, pre-flashed and VRM modded 4-port SATA card on eBay, which I installed in my QS G4.
It works out of the box in OS X - I installed 10.4, it's recognized as a bootable device when I hold down 'option' at startup, and it works fine and is pretty fast.
However, the...