Quit smoking! You’re healthier after one month, your lungs already start clearing up. And it will be easier to lose weight, you won’t be short-winded or anything.
Unless you’re morbidly obese, I would vote for quitting smoking first, for the reasons many posters have already mentioned. I like the solutions of the posters who combined quitting smoking with doing something physical for exercise (practical aversion therapy rules!). Regardless, discussing this with your doctor before you start is a good idea.
Quit smoking because its way, way, way more important.
If you want to avoid weight gain then try a few things to keep your weight down.
Eat a high fiber breakfast, this stabalized blood sugar throughout the day and cuts obesity rates.
http://www.newstarget.com/004603.html
Get alot of sleep (sleep deprvation screws up hormones that control appetite, causing overeating). 7-9 hours is recommended.
Eat a good deal of protein and fiber in general. The more the better.
If you do those things you should be able to quit smoking w/o gaining any weight, and maybe losing a little in the process.
So, we all agree it’s the smoking. Perhaps you should take up writing? 
Sincerely, it’d be a tragedy to lose a guy as talented as you. People like you should live forever; please get healthy and stick around a long, long time.