We currently run a lot of memory (16GB) and never approach even 50% usage. Regarding power, we use a 500W PSU and have to support a 3770K proc, 4 DIMMs, a couple drives, an nVidia GT 520 and a few cards.
We can calculate worst case power consumption (which never occurs). The GT 520's maximum power consumption is around 31W. The 3770K is tested to use around 160W (though this number is dubious). DIMMs we'll cap at 15W per module, so 60W. The mainboard we'll cap at 100W. We have three cards which cap at around 50W total. And then we have a DVD Drive which we'll cap at 20W. This is all worst case, which the computer almost certainly never reaches.
Totalling gives 421W. We have 10 USBs accessible on the case. If all of those are supplying 500mA, then 5V * .5A * 10 = 25W total power consumption (though this situation doesn't actually occur, and this doesn't factor in USB3 high amperage or any peculiarities in the amperage provided by these ports). This puts our system load at 446W at the worst case. We could get a 100W increase in our power supply but I'm very skeptical that the power supply limits are ever reached and actual usage is much lower, though depends on operation.
I'm surprised there isn't some absolute max spec for the draw on the USB ports. If there isn't, I'll have to assume the board itself is actually faulty. Thanks for the information.
Intel motherboards complies with USB standards, therefore each port will draw a max of 500mA.
For example if you are using a 1000W power supply, and you included 14 USB devices on the motherboard, since, each USB will draw 500mA at the same time, the max throughput may be affected, but the motherboard will maintain all 14 ports working.
Power supply shows maximum power utilization, however that value is not absolute, it is a nominal value. Therefore, the maximum power can vary about 10% to 15% less. That range could decrease if the PSU is 80plus certified
Please be aware this answer applies for any 7th or 8th series boards. Just in case you are using a 7th series motherboard with the I7-3770K processor you included on your post.