Also this resistor will cause the arduino to sink 2.3 mA through the resistor when the button is pressed. Even with a 4 buttons pressed, that's only 9 mA, which the IC can easily handle.
Actually when using external pull-up or pull-down resistors, virtually no current is being sunk or sourced via the Arduino IC. A AVR input pin is a very high impedance input, so the current flowing through an external resistor is being sourced via the 5vdc regulator IC, not the Arduino. It's only when an AVR I/O pin is programmed as a output pin, does source or sink currents pin specifications come into play.