It depends on serveral points.

- quality of your pci-e pins .. good quality pins can handle up to 9A per pin.
- wire guage and isolation of the wire, a 16AWG wire (wich is the maximun a pci pin can handle) with an isolation wich can handle up to 70C can handle less amps than an wire with an isolation wich can handle a temp up to 105C.

- type of conductor (coper or aluminium)

with 250W your on the save side, with 320W (almost 9A per pin) your pushing it (you will need good quality components).