It depends on the make/model of your psu and the graphics cards.
A normal GTX960 needs only a single 6 pin aux pcie psu connector.
A normal GTX970 requires two such connectors.
Some factory overclocked versions of graphics cards require more power than normal.
If your psu has the requisite connectors, you will be ok.
If you need adapters to get them, then that is not good and the psu needs to be replaced.