It should provide a decent starting point. For some of your components you can download the manufacturer's data sheets and provide a more accurate estimate. For example I'm using low power Seagate and WD drives which use about half the power alloted to 5400 RPM drives.
@Hank: Thank you so much! I tried to find a power supply calculator but none of the ones I found were any count. I didn't know NewEgg had one! Also, it never occurred to me to check out the original manufacturers' specs. Thanks!
@Mars: Thank you for the tips and extra calculator. Now I can average out the results.