PSU....700w or 600w....or 500w

well im thinking of building a new rig and ive gots all the parts i need, except the power supply, but i dont know how much i need. so how do you know which do you need? like do you go and add all the power consumption from all the hardware and subtract it from the psu or do you get the most power consuming hardware of your system and just get a psu that tops it or is equal to it?

thanx

yea i would go for the 700w but only if i needed it, but if i dont then im better off saving 20-30 bucks for a 600w.

here are the specs just incase you need them

E6700               8800GTX

Corsair 2gb        X-Fi

Seagate 320      PSU....

EVGA 680i