Making Faces same height and then merging?

Say I have two rectangular prisms side by side, one is taller than the other. How can I make the height of the shorter prism the same exact height as the other and then merge the edges on top that are touching to make one edge? I’m trying to make a house basically and need to do clean measurements. Thanks.

Edit: Using Blender 2.5 Beta.

There are a number of ways to do this
Select a top vertex of the height you want, Shift+S / curser to selection.
Change the pivot point on the 3d view header to 3d curser.
Select the edges you want to move to the height of the 3d curser, S (scale) / 0 zero) enter.
You can also use the snap tools to just move the edges and snp to a vertex at the same height you want to move them.
Probably not a clear explaination.