|You can never have too much power, ever. It just costs more.|
Just enough is not enough.
Enough with some in reserve is what you need, therefore err on the side of too much.
1000 watts seems like a bit of overkill unless yo are running a corporate server.
720 watts should be adequate for a desktop.
However, the wattage doesn't mean much in isolation.
It is largely used as a marketing gimmick and you should be looking at the amperage on the different voltage rails. An unscrupulous manufactures can put a high amperage on the 3.3 rail and lower amperages on the other rails and still have a total of 750 watts output. Whereas you really need the high amperage on the 12 v rail which is a little more expensive to implement.
Quite often an expensive 500 watts power supply will out perform a cheap 750 watt power supply.