Look up the specifications of your power supply. Even if you don't have a 6-pin pci-express connector, you can get an adapter that connects to two spare 4-pin molex connectors. However, you should also check how many amps the psu provides on the +12V rail.
Most GPUs that require extra power connectors include an adapter in the box for power supplies which don't have the 6 pin connector built in. It's an adapter with two four-pin MOLEX connectors leading to one six-pin GPU connector.