Hate to sound like a broken record, but ask the HD before going the fountain soda route. Additional ice storage/Ice maker, and possibly needing additional counter top space may be an issue in your area. Also, the ROI on the additional equipment vs the savings/margin of fountain drinks (lids and straws). Don't forget about the time/labor to scoop ice, fill soda, put lid on and get to the window.
That being said, in some parts of town, Pepsi, Sprite and Orange are big. In others, it's a Coke, Dr Pepper, 7 Up deal. We've gone the route of selling what we sell - Cans of Coke, Diet Coke, Sprite and Water from either Sam's or Costco depending on price. We consolidate our inventory, and can always get product, and never worry about being out of CO2.