Under Blendre it's pretty simple :
-Select your object
- Enter in edit mode
-Just select the faces you want to smooth
- then having the mouse anywhere in the 3D view :
press "Space"
It will onpen the menu , then choose the submenus :
- Edit -> Faces -> Set smooth
or if you want some faces (or all) to be not smooth but solid:
Choose
- Edit -> Faces -> Set Solid


If you want a model to mix smooth and Hard edges in game,
the way to go is like already said here:
Apply one texture on the faces to be smooth and another
on the face to keep solid, then apply two different shaders
to the two textures.