The Raspberry Pi runs by default at 700MHz. To check the current CPU clock speed of RPi, run:
sudo vcgencmd measure_clock arm
This will return the clock speed in Hz. On my RPi, this is what was returned:
To overclock the RPi, edit
/boot/config.txt and change the arm_freq line to your desired clock speed (in MHz). Make sure that the line is uncommented by removing the leading #.
Reboot to see it take effect. Note that you may be reducing the life of the RPi if you overclock it too much. Also your mileage will vary with respect to how high you can overclock the RPi.