To clear up a few things:
@siccoyote : Your dad's right. The 7805 has to "get rid" of the extra voltage between the input and output voltage (7V difference). The 7805 does have a certain amount of overhead that it needs to be stable (usually about 2V or 3V above the output voltage) so it wouldn't work with a 6V supply, so the most efficient supply for a 7805 would be about 8V. His math's are also correct: Power = Current x Voltage (P=IV) The extra 7V x 0.5A = 3.5W. With the 9V supply the 7805 would only need to sink 2W of excess energy.
@geebus: Amps are like beer at a party. Just cos you have 100's of litres of it, doesn't mean you have to consume it all (or at least you shouldn't). The amps on a PSU tell you what it can supply, the circuit decides how much it's going to take. So theoretically you could use a 50A PSU on the CPC, but there is an associated risk with doing this. The CPC rarely needs more than about 2 or 3 amps and the traces on the PCB are sized accordingly. If you are using a 3A PSU on you CPC and one of the RAMs gets a short circuit (a common occurance), then the CPC will pull as much current as possible (3A in this case) and the RAM chip will get hot, normally no further damage should occur. However, if you were using a 50A PSU and a short occured, then the current could theoretically rise to 50A (in reality other factors would limit this) and you would fry all the traces off the PCB, something that is extremely difficult to repair.
So much like the beer at a party. Make sure there's enough at hand, but don't over do it.
Bryce.
