What is the maximum current the Arduino ADK can supply through the + 5 volts?
The Arduino Mega uses the RYK 117-5 5 Volt regulator which from the references I was able to find has a maximum current delivery of 800 mA. This of course depends on your input source and the amount your loadboards and mega itself draws.
Keeping in mind that your Mega itself can draw as much as 100 to 200 mA depending on what your Mega is coded and wired to do. In general it is safe to design to run less than 500 mA externally for a shieldless board, just be sure you have a power source capable of providing that (less than 200 mA is usually available on USB).