EA-500D has two +12V rails of 22A (264W) each. I would advise against using Molex->PCIe or SATA/PCIe cable. Presumably the unit is designed for the graphics card to be powered from a different rail than the processor, and when you use an adapter, you risk powering the GPU from the same rail as the CPU which can result in overloading that rail. Time for an upgrade.
If you're in the US:
Antec HCG-620M $60 after rebate. 2 x 6+2-Pin connectors, single 576W rail - it powers any single GPU graphics card except some custom cards that need 3 connectors (MSI 290X lightning for instance).
What CPU do you have? It could be a serious bottleneck to the R9 280, or it could be fine.