So what's the deal between these gold, silver, and bronze ratings.
Those ratings indicate efficiency as tested by 80plus. You can read about it in the link below, but currently there are 5 levels of efficiency labeling...80plus, Bronze, Silver, Gold, Platinum, and each requires a higher level of efficiency from a power supply than the level below it, with simple 80plus cert. being the lowest and Platinum being the highest, or most efficient.
http://www.plugloadsolutions.com/80PlusPowerSupplies.aspx
One thing to remember is that the 80plus testing is done on room air, not in a hot box environment and more than a few power supplies have been shown to struggle to meet their rated efficiency levels in a heated environment.
Then there's the issue that some unscrupulous power supply manufacturers will send in a ginned up version of a particular power supply for evaluation, but the retail versions never meet the results of testing on the "special" one.