pcb design - PCB tracing - Ground plane and Power plane?

Is it generally a bad idea, in simple or moderately heavy circuits, having two layers, to have a power plane in the top and a ground plane on the bottom?

The tracing would become much easier, there wont be virtually any noise, better heat dissipation and aesthetically the circuit would be layed out neatly.

Also, is there any handy guide to refer, when to use a thicker trace for different values of current?

Thanks.