The following stats are based on an overclocked cpu and a single video card at load:
Overclocked Quad-Core CPU - 130 watts
Motherboard & chipsets - 60 watts
2 Memory Modules - 6 watts
2 Hard Drives - 20 watts
optical drive - 20 watts
9600 GT at full load - 85 watts and 7 amps
Total Power: 321 watts
Since you are not going to overclock the cpu you're requirements are a little lower. The wattage for a single 9600GT is fairly low and varies depending on the model and memory. I listed the higher 9600FT wattage.
The source of the information is a recent Tom's Hardware article about actual power requirements for video cards. It also included power requirements for other components:
http://www.tomshardware.com/reviews/geforce-radeon-power,2122.html
Manufacturers have a tendency to overestimate total requirements, especially considering what hardcore gamers into serious overclocking do to their systems.