Best to paint ceilings when you paint the rest of the room, otherwise either the walls or ceiling will look grimy. Yes, painting ceilings white can make a big difference in how a room appears.

Agree with apple pie that it's best to wait until you're changing the walls too. It's hard to say a color, but if you have white trim use the same color as that for the ceiling in a flat finish.

I say yes to painting ceilings when the rest of the room is done, but I don't think you should pay a premium price for specialty colors like white dove. Ceiling white is fine unless you are doing a color like blue.