Okay, a 900W PSU will be able to provide up to 900W of power (as long as the manufacturer hasn't lied about its wattage).
The Bronze, Silver, etc ratings are the PSU's efficiency: how much power it drains and how much it provides. Basically, it measures how much energy is lost when it is being "processed" by your PSU.
A 900W PSU with a 90% efficiency would mean that it can output 900W, but to do so it'll be using up 1000W from your outlet. It's using more power than it provides, and the wasted energy is let off in the form of heat.
This of course means it's better to have a higher efficiency: it'll save in your power bill, and it'll reduce your heat emission.
Also, be wary that the efficiency varies depending on the PSU's usage. Here is the graph for your PSU: http://www.pcper.com/images/reviews/1040/32-Efficiency-graph.jpg
So, your PSU will do just fine.