the 5770 needs less than 30A (20 should do) on the 12v and AMD recommends a 450w psu. A 500w antec is more than enough for it.
artifacts can occur due to a lot of things, mainly a driver issues, Insufficient voltage or a dying card. A psu will generally keep the voltage right but current could be insufficient, just check the voltage on the 12v with a program to make sure it is about 12v. Drivers probably isn't your problem but you can try reinstalling drivers.