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It would be better to not use the 12v gamecube power supply. It will be a bit more complicated, but I think the gamecube needs 3.3v and 5v, correct? You can get these voltages by means of TI switching regulators (look in the FREE DC-DC converters thread in the N64 forum)
it can get the right voltage but not enugh amps for GC. what you should do is ise this to get 12, and search "5vout step-down isr" and "3.3vout step-down ISR" to get the other voltages. get one that looks the same as the 12v one because i know they work. the reason you want to do this is because it's more efficient then stepping it up and then down again, you don't need a 2A regulator, and those are easy to wire.