Dont go for price over quality. if someone is selling a 800w for cheaper than another firms 650w then its becasue its wa*k basically. ive got the corsair tx 650 which would be plenty for you if your are planning to OC the cpu and even the card. i had a 5770 on a 500w supply on stock cpu and gpu settings so 650w good make like corsair will be perfect. only buy over 650w if you are planning on future upgrades, otherwie itl be wasted.
for copmparison i run a 5970 with te corsair tx650 at stock cpu and gpu settings and thats plenty so with a 5770 youl be fine.
Here are the official ATI power supply requirements for the ATI Radeon HD 5770 and 5750 video cards:
450 Watt or greater power supply with one 75 watt, 6-pin PCI Express® power connector recommended
600 Watt or greater power supply with two 75 watt, 6-pin PCI Express® power connectors for ATI CrossFireX™ technology in dual mode
The recommendations are for an entire pc system.
Corsair and Seasonic are two brands that have a reputation for high quality power supplies that consistently earn high marks in technical reviews. They are reliable, stable, and come with a 5 year warranty. Some of the newer models come with a 7 year warranty (in the USA).