Can I turn the power supply voltage (5V) on and off through code,
what is the voltage that comes out of the digital pins when I use digitalWrite(pin, value)?
Is a breadboard too weak
I think I'll be using the IRF510 from radio shack
as well as a resistor (100-200 ohms) from gate to ground, is recommended for mosfets
And what is the protection diode for?
using the IRF510 from radio shack..seems like a popular one and it fits my project).
2 questions here.. If I am controlling the voltage from the Arduino's digital out, which should power off the mosfet at will, why do I need a resistor? Does it somehow shut it off faster?
You don't need it. It is only needed if you want to blow up your mosfet.