As far as the specific power goes, you should consult your hardware documentation. However, as a general rule, use these numbers....
Component Watts
AMD Athlon 64/64 FX 90-110
AMD Thunderbird 50-60
AMD Athlon XP 50-80
AMD Duron 40-60
Intel Pentium 4 60-100
Intel Pentium III 25-40
Intel Celeron 30-60
Basic Graphics Card 25
High Performance Graphics Card 35-100+
Motherboard 18-28
Optical Drive (ea.) 25-35
Hard Drive 25
Memory Module (ea.) 7-12
Unpowered USB/Firewire device 5-10
PCI Sound Card 5-10
Additional PCI Card 5
Floppy Drive 5
Fan 3
Keyboard and Mouse 6
Typically if I can't find the exact for whatever reason on my hardware, I take the average, add them up, and try to give a safety buffer of 30%.
Hope that helps.