Totally depends on you, and how much you use your stroller now. My kids are 17 months apart and then 23 months apart, so DS1 was just over 3 when my DD was born. We did buy a double stroller when my second son was born. We lived on the west coast and used our stroller a LOT, since the weather was always so nice. Chico does make a nice double stroller that would use the car seat you currently have, so if that is what you want to do, you could just replace part of your travel system, and not the whole thing.
However with my third we simply went back to the single stroller and replaced the carrier, they make these lovely straps that attach to a wrist or a stroller and have little places for kids to hold on to them, I got two, in different colors and each boy had a strap to hold when we went out. If they didn't hold on, they didn't go out with us, period. it kept them close and worked for walks around town, since we were living in OHio then, and didn't go out nearly as often. once she was big enough we bought a wagon, but i only once had any issues with my boys walking and behaving well, and they never complained or wanted carried.