That sheds some light on the issue. COmpanies like Ibuypower and cyberpower will shamelessly include garbage non-name PSU with thier PC as standard to cut costs. These power supplies are as cheap as they come, I pretty sure that the 480W rating is fake as well (most of them are 300W CPU that have been re-branded).
So not only do I think you might run into problems with that PSU, it might even be a danger to the rest of your computer (and the PC company also has lackluster warranty support).
Honestly I would throw out the standard PSU and put in something good in there, like 550+ watt unit from Corsair or Antec.http://www.newegg.com/Product/Product.aspx?Item=N82E16817371044
something like that will be able to power anything short of 590 GTX or a 6990HD
If money is really short, this PSU will be abel to power your system + 5770 no problems and its very cheap after rebate+promotion. http://www.newegg.com/Product/Product.aspx?Item=N82E16817139027