Unfortunately, the IRF510 is not a logic level mosfet, it's designed to be driven from 10V on the gate instead of the 5V that the Arduino provides. With 5V on the gate, I wouldn't ask it to pass more than 500mA. Look for a logic-level mosfet instead.
Unfortunately, Radio Shack appears not to stock any logic level mosfets in its stores. You're not alone, here in the UK, Maplin is equally useless.