I would recommend against going with two separate PSUs. I think you need a decent, high-wattage unit to mod. I built a smurf back in the day and the HD wasn't recognised when on the built in IDE bus but a separate PSU (from an external enclosure). Last week I encountered the same phenomenon with an MDD and a drive running from a SATA PCI controller.
I have run MDDs with 5 drives in them on the stock PSU with no issues. One was a dual 1.42GHz, the other was a single 1GHz upgraded to dual 1GHz. If you have an older G4, it would seem fair to assume that it will be drawing less juice than either of these. Going by memory, none of the MDD PSUs exceeded 400W by much.
Based on this, I would say a 600W unit would be a pretty safe bet. With this many drives doing a fairly important job, it might be worth forking out for a more expensive, high quality model.
If you want to take the time, you could look up the power consumption of the drives you have your eye on. And I'm sure you can find the consumption of your G4 chip too. I believe Seagate have recently launched some low power drives too.
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