i already have 1 5770 and looking to buy another but the box says it needs 650w of power to run them in crossfire, but the thing is i have found a psu that i would like to get which is a Coolermaster RealPower 620W Modular PSU will this power them along with the rest of my system. i found a website that calculates the power that your system needs eXtreme Power Supply Calculator and it says that 550w is enough with a 35% capacitor aging.
AMD Phenom II 955 Black Edition
Asus M4A785TD-V EVO
HIS Radeon HD 5770
Akasa AK-968 CPU Cooler
Seagate 500GB Harddrive
LG Supermulti DVD drive
Antec 120mm TriCool Green L.E.D. fans x2
Antec 120mm TwoCool fan
Antec 140mm TwoCool fan
Official power requirements are always a bit overrated, i was running 5770 Crossfire with i5 750 on 500w thermaltake PSU for months without any problems but to stay on a safe side get quality 550-600w psu.
Take a look at this one its on sale now, it has 5 years of warranty also: http://www.newegg.com/Product/Product.aspx?Item=N82E168...
make sure you check the amps on the 12V rail of your PSU (says on the side of your PSU). PSUs are often overated as far as wattage, as all the important components use the 12v rail (or rails). multiply the amps on the rail by 12 to get your actual usuable system wattage with that PSU, if you have around 500+ watts total on that rail, you should be fine i would think, permitting you don't overclock.