Looks like it could be an (old) ATX 1.x PSU. Also, they're lying bastards. 500W? Not even close. You'll need a new PSU.
Yea... No matter how creative I get I can't get above about 260W. Ballsy even for a PSU maker. If that Web address is navtech.org it goes to a mobile phone maker lol.
Even if you don't upgrade the GPU I would STRONGLY recommend replacing the PSU ASAP