Check out this slightly out dated power usage chart and calculation from PCPowerAndCooling.
That should allow you to get a rough idea of your power needs.
For overall power supply wattage, add the requirement for each device in your system, then multiply by 1.8. (The multiplier takes into account that today’s systems draw disproportionally on the +12V output. Furthermore, power supplies are more efficient and reliable when loaded to 30% - 70% of maximum capacity.)
CPU: Intel i7 2600K @ 4.6Ghz
Mobo: Asus P8P67 Pro B3 Revision
Mem: Corsair Vengeance 8GB (2x4GB) DDR3 1866MHz
Video: 2 x MSI HD5850 Twin Frozr II (CrossfireX)
HDD: Crucial C300 128GB SSD, Seagate Momentus XT 500GB
Sound: Realtek® ALC892 8-Channel High Definition Audio
OS: Windows 7 Home Premium 64bit