Hi,
With significant power being switched, I recommend getting the relay(s) physically away from Arduino (Close to the load if possible), and using Optical Isolation so you don't need any direct electrical connection between Arduino and the relay circuitry.
See the schematic diagram on this page: http://goo.gl/PcWJ2 You could do something like this yourself, or buy relay boards. (The one shown is only $4)..
Read up on controlling power with Arduino on the ArduinoInfo WIKI here - http://arduinoinfo.info
See: http://arduino-info.wikispaces.com/ArduinoPower
DISCLAIMER: Mentioned stuff from my own shop...