http://extreme.outervision.com/psucalculatorlite.jsp
http://www.tomshardware.com/reviews/power-supply-protection-calculate-consumption,3066.html
Motherboard 50 W–75 W Depends on the number of integrated components.
Processor 25 W–150 W For each physical processor (not cores). Most are 50 W–100 W.
RAM 5 W–15 W For each module (DIMM).
Integrated video 5 W–15 W Integrated into the North Bridge chip (Ed.: Though, increasingly on the CPU).
Discrete video card 25 W–300 W For each video card.
PCI card 5 W–15 W For each nonvideo card.
PCIe card 10 W–25 W For each nonvideo card.
Hard disk drive 15 W–30 W For each drive. Power use increased during startup.
Optical drive 15 W–35 W For each drive.
Cooling fan 3 W–5 W For each fan.
USB/FireWire 2 W–5 W For each used port.
With 9.5 jonnyguru rated PSU's available for $60-$65 I really can't see the sense in getting anything smaller. PSU's hit peak efficiency at 50% load....so if ya save $8 on the purchase, you'll get that back in electricity savings. That doesn't mean buy a 1200 watter when ya need a 650 as that's not a small price increase but in the smaller sizes, it's just not worth the savings, at least if limiting the search to quality units.