Yes, it is compatible. If your current power supply is 550W that is enough, but just make sure it has the right connectors for the graphics card. It needs two (2) six-pin power connectors, so check to see if your current power supply has those.
I'm sure it has at least one if it doesn't have two. In that case you can use a molex to six-pin adapter that's probably included in the graphic card's packaging.