^Yes and no. If the PSU doesn't have PCI power connectors, you can use adapters, but ONLY if the PSU is otherwise capable of providing the required power.
What brand and model is your "600W" PSU? If it is something like a Corsair or OCZ (not sure Seasonic and Antec have a 600W model right now) you should be fine. If, otoh, it is a Diablotek, Logisys, or other Chokemax brand, it will likely only be good for 60% of its label, and will die under the load from a high-end graphics card (possibly taking other parts with it).