Have you connected the 6 pin power connector from the PSU?
What is the make model of the PSU being used, you should have a 6 pin lead coming from the PSU.
Also did your gpu come with any adapters in the box.
http://www.overclockers.co.uk/showproduct.php?prodid=CB-018-AK&groupid=1929&catid=153&subcat=
Totally forgot the 5770 had a external power!!![]()