I agree - giving up smoking should come first.
I smoked for 25 years and recently started to feel very unhealthy. I couldn't even walk up a flight of stairs without huffing and puffing - terrible.
Concerned that I'd put on weight when I quit, (I was already overweight) I started eating healthily 3 weeks before my planned quit date.
Since then, I've switched to W8 and to date, I've not smoked for 3 months, have lost 24lbs and I FEEL GREAT! (I also quit using Champix which helped incredibly, but I did suffer some nasty side-effects).
Good luck to everyone trying to quit - it is extremely difficult, but I believe if you really want to stop, you'll do it. And like others have said - just think of all the cash you'll save!