card manufacturers over estimate PSU requirements. what is the more important aspect is what amperage is on the 12 volt rail, that is what supplies the power to your important components such as the cpu, graphics and the HD motors.
generally with a single 6 pin power connection on the graphics card; a PSU that provides at least 24 amps on the 12 volt rail is fine.
Well that's great, but the HDD's, motherboard and CPU and other minor hardware doesn't draw too much power?
components such as RAM, motherboard chipset and the logic for hard drives get their power from the 3.3 and 5 volt rails. the chipset is 8 watts for intel and up to 15 watts for AMD. ddr3 RAM is 4 watts a stick and logic boards on the hard drives is a few watts.
the main components are the graphics card, cpu, hard drive motors and fans on the 12 volt rail.
gpu: that 7770 upto 100 watts
cpu: upto 100 watts for a sandy/ivy and up to 150 watts for an AMD/sandy bridge-E ( overestimated with overcloking in mind)
fans: 3-5 watt each.
HD motor: 10 watts spinning up @ boot and 7 watts read/write
so a 24 amp 12 volt rail @ 288 watts will cover most anything with a 7770 card.