How To Calculate The Power Supply And Check How Much I Have

Intel DH61WW motherboard

Intel Core i3-2100 3.10 GHz

4GB DDR3 RAM

nVidia GT 430 2GB

And I want to install a DVD-Writer, a webcam and a printer in future. How can I know the power supply needed when all of these components are performing at highest possible limit and how can I know how much power supply is installed in my PC now?