U need to caculate from every componants, all branded

componants can get the info from their website. example: a 160GB seagate hdd is 9w, a 200gb+ seagate is 13w.
the quickly way to estimate it is to find a same spec pc from brand name like HP, then u should choose power supply not less than what they use there as custom pc will need more power.
make sure u don't fool by "true" power and "max" power. a power supply marked with 400w max has no meaning as the stable output is from 50%max to 80%max depend on quality. take a look at antec power supply will give u more info