Okay, I think some people think about power backwards. Which may give the OP the wrong impression regarding their question. "Is it okay to use?" The answer is yes.
Power isn't pushed. If you have a 1000watt power supply it's not pushing 1000watts of constant power to your components.
A 1000watt power supply means that it can safely supply 1000watts if it's asked to.
The way electricity works is that it's pulled or drawn by the component(s) that require it.
This goes the same for all electrical devices. This is why your 100watt light bulb doesn't explode when it's connected to a nuclear power plant that is supplying 4,000 megawatts.
In fact it will do damage when there is under supply of power.
In short, as long as all your components add up to use less watts than your power supply, it's 'okay' to use.
Using this calculator below will give you a good indication of your power draw. http://www.extreme.outervision.com/psucalculatorlite.jsp
However, there is a few things to consider when buying a PSU.
- Running a PSU at 100% of its rating will be like running as fast as you can on a treadmill. You can do it for a while, but eventually you will fail from fatigue. - A power supply will vary in energy efficiency at different wattages. In your specific case, it looks like your PSU is most efficient at 400-500w
I say it would be a great purchase. And future proofs you. A good PSU will last 10 years.
Source: http://www.techpowerup.com/reviews/EVGA/SuperNOVA_P2_1000/6.html