OWC is a nice option.
I bought a 250 GB hard disk from OWC and their software for my Sawtooth PowerMac G4. Works like a charm! I now have a 127.87 GB partition and a 104.77 GB partition after formatting with Disk Utility.
If you go external with a hard disk enclosure and firewire, I don't think you need the extra software. External drives >128 GB will be recongnized at their full capacity. I believe that only internal drives have the 128 GB limit on the older machines.