The guy that told you that you need 750 watt power supply for a GTX 560 has no idea what he's talking about.
What brand is your current power supply? What model is it? If you cannot find that info, could you take a picture of its label and show it to us?
The GTX 560 Needs two 6-pin power connectors from the power supply unit. If you power supply has them available, it probably is enough. However, if it doesn't have them, it's probably not enough. It still depends on what the label says though.