The wattage that a PSU is rated is how much it CAN provide, not how much it WILL provide. So if you have a setup that uses 300W, the 650W PSU will give you 300W. You can go over as far as you like because the PSU only supplies what is needed.
As for the 650W for $30, I'd be careful. What brand is it? Post a link for us to check out. Cheap power supplies are typically pretty crappy.