I bought 2 160G drives, and 1 320G drive, and when I started working with them realized that the G4 has a 128G limit on drives, because the on-board controller does not support LBA48. (Bummer!) (G4 AGP graphics)
To be clear, the controller does, but the drivers available to OF don't. Just put ofwboot.xcf and the boot kernel into an HFS partition at the beginning of the disk and you'll be fine - as NetBSD boots, it'll load its own driver and will see the full size of the drives.
Running ATA or SATA cards is another issue altogether because you have to find cards which are supported under NetBSD and bootable via Open Firmware. I use a number of ATA cards in various 7600 type machines which are doing NAT and such, but they also have the limited-to-128-gigs-until-kernel-boots issue as well.
Just FYI, if you want to use SATA drives the easiest way to do it is to put them on the main IDE bus via SATA to IDE adapters.
If you want to boot without the kernel on an HFS volume, you could always buy a CF to IDE adapter and put ofwboot.xcf and the kernel on a CF card which has another drive set as the root filesystem.
I've done all this before, so I can say with certainty any of it will work.
John