I can't think of any 600W graphics cards. ~400W is where they top out.
It's going to depend on the psu. Some are great and are underrated. Meaning this 550W psu can output 700W safely and you won't noticed anything. Some are set to output 600W and shutdown at anything over that. You'll be fine in that case, though you might get odd shutdowns if you ever hit 601W. Others can barely handle the 550W they claim to be and will shut down as soonas you load the game. Others still are fire hazards and lie about being a 550W. These should be avoided at all times.