The furniture often 'dictates' what you can and cannot do (if you already have something and can't replace it), but it is possible to work around that by sort of 'down playing' the furniture style. The paint would then be at the top of my list because the color of your walls has such a visceral effect on you. The lighting is nice to work with but since people on a budget cannot go crazy with special lighting (recessed lighting for instance) it is not one of the first things I think of. For me....right up there with paint color...is the wall decor. THAT is what really makes the 'theme' in the room right? That is, if you mean 'ambiance' as in mood/style/feel. If it is simply a matter of making a room 'cozy' then turning off most of the lights and using some well placed lamps will do it most times.
