Re: Great.. .but better ways to do the power...
A PC power supply will be VERY unhappy supplying many amps at 5V and none at 12V. It may refuse to work with that load, overheat and emit smoke, or simply waste a large fraction of the power going in. They're designed for use with modern PC hardware, with the lion's share of the power being consumed at 12V.
Just source a single-voltage power supply that delivers enough amps at 5V. There will be plenty of PSUs to choose from at RS or CPC. It may be cheaper to use multiple 5V 4A or 5A "bricks" than a single (say) 40A unit, and may also be easier to wire up.