Specifically, on some Gigabyte motherboards, among them the MA78GPM-DS2H which I have, the USB ports don't work if you use an Enermax Pro 82+ power supply of the kind I just bought. Which is inconvenient, since on that particular system both the keyboard and the disc with the OS on were attached by USB.
The wattage of the PSU is not the issue here, mine was the 625W version, which would be enough to melt a USB stick let alone power it. It's not a matter of damaging the motherboard; when I put the old PSU back in it started working again.
I now have everything set up sensibly. It took me all morning. Particularly annoying was the moment that I discovered that the video card I was trying to install was half an inch too long to fit in the case I wanted to install it in, and so I needed to swap two motherboards round (that is, dismantle to total bareness two cases filled with fiddly electronics held in by multitudes of small screws and connected by python-like bands of cables, and reassemble the other way round). At least it's not covered in grease, you don't need a hammer to loosen it, little of it is particularly sharp, and there is no risk that it will disintegrate while I'm relying on it to keep me from driving into a lorry at 70mph: I am not one of nature's garage mechanics.
A discovery that may be useful to other people: the little hexagonal stand-offs for attaching motherboards to cases are not standard either in diameter or in top or bottom thread pitch between case manufacturers. So if you have mixed the bags that came with two cases, you have to try at least two screws in each stand-off to see which fit, and if you need to fit extra stand-offs you will have to try lots to find one which fits. The stand-off most inaccessible on the motherboard will, of course, be the one in which you have to try most screws.