No problem. They always put a PSU with a wattage much larger than what is needed to account for the crap PSUs out there and leave plenty of headroom.
As you can see even an overclocked 280 does not break 400W under load from a high stress benchmark.
I would avoid the 500B, it is a mediocre unit and is not the best choice for a gaming pc. Do you have a budget for the PSU? Before or after rebates?