oh right , my bad i didnt realise you had another thread ,generally i know cos ive done it before
although you could do it systematically by say adding up all your components
working out how much each one uses ,
say like these are just power levels for estimation and most arent actually factual
CPU-45W
Motherboard -50W
HDD1-10W
HDD2-10W
DVD-10W
GPU-250W
that comes to 375W , i would then add on about 50W just for good measure , in this case i would get a 450W
The eXtreme Power Supply Calculator Lite v2.5 is legit and works very well. There are many different calculators that will all work pretty well in helping you decide on how much wattage you'll need.