Wht is minimum Amp on +12 V rail required to run ATI Radeon HD6850 or HD5770...
For single reference version graphics cards:Radeon HD 5770Maximum Board Power:
108W (9 Amps from +12V rail)Idle Board Power:
18WPower Supply Requirement:
450 Watt or greater with at least one 75W 6-pin PCI Express power connectorRadeon HD 6850Maximum Board Power:
127W (10.6 Amps from +12V rail)Idle Board Power:
19WPower Supply Requirement:
500 Watt or greater with at least one 75W 6-pin PCI Express power connector
You need to take into account that the CPU will be getting all of its power from the +12 Volt rail. The motherboard's on-board devices, expansion cards, hard disk drives, CD/DVD drives, cooling fans will also obtain some or most their power from the +12 Volt rail so they must also be included in the requirements.
You don't want to be running the +12 Volt rail(s) at its maximum output current continuously. Some of the poorly designed power supply units will generate high levels of ripple voltage on that rail when you draw the maximum rated current. Excessive ripple can destroy your system components.
I will also recommend 30 Amps or more as a safe minimum on the +12 Volt rail(s).