It is always better to have more wattage than you need. I wouldn't recommend pushing more than 80% of your PSU capability. A 1000kw supply should work, but you are going to be pushing it hard. This will effect its lifespan. So it really come down to how much you want to gamble. Is it more important for you to keep the initial investment as low as possible or to have something more reliable?

Keep in mind also that you can't always count on manufacturers to supply accurate information, so stick to PSUs with good reviews from well known companies.

-SteveA