I would use the rule that each board you use will pull 60 watts. I know some board pull less then this (pentiums) and some quite a bite less than this (P-2's/Celerons) but that is what to figure it at. Plus you don't want to run each power supply at FULL power.
Any more questions feel free to ask.
LD
PS, if you have a watt meter, you can customize it quite a bit more. That is how I came up with that rule. I measured many machines running at FULL cpu, and then found out howmuch a Power supply can really give. Moding power supplies TAKES TIME. It's not a quick process, and if you make a mistake, it will cost ya. SO test the power supply with a multi-meter before you try it on motherboards.
Good Luck