(from what I've seen of diagrams it is really easy and I think just a rewire and a fuse or something added?).

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Easy enough, but there's a little bit more to it than that.

Their is a site that sells pre modded cables.

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There is, but I am personally very dubious about the wiring scheme he's chosen to supply the standby power (to the +28V pin)

You can read about my successful adventure here.

Short version:

• Use an ATX power extender cable for your rewiring, so you can swap in another ATX PS whenever you like.
• Spend $10 on the tool to remove the pins from the Molex connector, because it's a PITA otherwise.
• +5VSB (standby) from the ATX PS works fine on the +28V pin, at least on my machine, but you won't be able to use powered Firewire devices without a powered hub, nor ADC monitors.
  
• If that doesn't work, a Powerbook power supply (+24V) is worth a shot.

Best of luck and please do let us know how you progress