Max current through an Arduino Mega?
You're on the right track here. In the datasheet for the ATmega1280 I find this box:



What you want to look at is DC Current Vcc and GND Pins  ....  200.0 mA

This is the total limit of all the current going through the chip (because everything must come through Vcc), and it is the absolute maximum. Don't go near it. You need to boost this with some transistors.