|"my old card (gs8300) PCIx also works perfectly"|
PCIx & PCIe are 2 completely different things. I doubt you have PCIx. And your old card is probably an 8300 GS, not a gs8300. It's important to get the terminology right.
"I have 600watt Power Supply Cooler Master"
Cooler Master makes a lot of different PSU series - Elite, eXtreme, GX, etc. The eXtreme series is very low end. If that's what you have, you'll find this interesting: