I have 2 sticks of 256MB PC133. Would having 1 stick of 512MB PC133 offer less power consumption than 2 sticks of RAM but of equal total capacity, and would it offer the same power consumption as 1 stick of 256MB?

It depends, if the 256MB sticks are singlesided it will not have any difference, but if they are doubblesided you will have 1/2 the ammount of memory modules if you go with one 512MB stick.