How to find total power requirment

Is there any software to find a actual power requirement for my pc.

for example. my confg is
AMD 64 Athlon 3500+
Asus CSM series motheboard
2GB DDR
7300 GT 256 MB
80GB WD
500GB WD
2 mp. Web Cam
Headphone
Sony DVD Writter

currently i have a 400W (labelled ) PSU..
how to calculate the actual power requirement for above config.
If there is any utility to find, it will be more useful to decide for replacing my PSU.