Congrats for planning to give up smoking - you won't regret it! I gave up smoking about 18 months ago now and was also dieting at the same time, so I'll tell you a little about how it was.
Dieting at the same time did not make it harder and the exercise gave me something to focus on and keep busy with! I'm sure you know that giving up can be difficult - but it wasn't as hard as I thought it would be , infact the hardest thing was committing to stopping and then actually DOING IT!
I knew I would need something to do with my hands, so prior to giving up I decided to start cardmaking and stocked up on lots of stuff to keep me busy!
Foodwise, I did not turn to food out of boredom, BUT I found I was alot hungrier. I found out that this is completely normal because smoking is an appetite suppressant, so when you quit you're natural appetite reappears. Also I found out that smoking increases your meatabolism!
So, consequently I did gain about 6- 8lb during my giving up smoking - I know people who say that their weight balooned when they gave up, but if you make sure you don't use the cookie jar as a cigarette substitute, then any weight gain will be minimal.
Before I gave up I was determined not to gain any weight at all, but as time progressed I realised that giving up smoking was the single most important thing I could do for my health - and if it meant gaining a few pounds then so be it.
The best thing about giving up and dieting at the same time is that you will really notice a dramatic improvement in your fitness - the difference is amazing!
Hope this helps - good luck! Feel free to PM me at any time - I'll gladly support you through it!
Love Amanda x