If you really want soda, the best would be something like ginger ale, or caffeine free versions of
regular sodas...but if you want
regular soda, you can have up to 250mg of caffeine a day, and the sort of soda you drink doesn't matter so much as how much caffeine each serving has...for instance, a 16-oz coca cola has 45mg of caffeine in it (just checked the bottle I was drinking ;-) ) so you could have quite a few without risking drinking "too much caffeine".