Dont neccessarily go for the highest rated power output PSU. The reason being that peak output can be very different to normal output. You would be better off buying a lower rated peak output model at the same price because it would be more effecient.
For example a $100 PSU rated at 1000w may actually only run at 600w 90% of the time. Whereas a 800w rated model for the same price may run at 700w 90% of the time because it has better tolereances due to better manufacturing and components.
Of course you should look into what psu your going to buy because this rule does not fit all, you may even find it better to get a decent second hand PSU and save some money but get a better model. Also a more expensive model will be more likely to last longer.
Just thought I would mention my view...
dont ever buy a PSU that isn't 80 plus certified. the 80 means at it's worst it will operate at or above 80% efficiency