What size power supply do i need?

i am upgrading to a intel i7-870 with 4 Gig ddr3 1600-2250 Mhz on a gigabyte GA-H55M. with it i will also be using a 30gb SSD, a 1TB WD HDD and a ATI HD 5670. i currently have a ATX 850 WATT power supply. i wanted to know if that would be a big enough PSU to run the computer. also would it be enough to run dual HD5670's. but i mainly want to know if it will run just 1. thanks :)