I am so glad to read everyone's thoughts, we are in this debate too!
I love the way our 2 kids get along, and like a PP said it feels like the world is made for families of 4... but DH and I also are not sure we feel done. I go back and forth. I am the oldest of 3 girls and we had a lot of "two against one" that never got resolved even now... but there were other issues at work too so I tell myself I can't decide based on that experience alone. However, I do wonder if we go for three, maybe we shouldn't just plan on a fourth too. :LOL
Anyway, I told DH that I wanted to decide for sure between DD's 2nd and 3rd birthday because that is the earliest and latest I would ideally want to get PG with #3. She and DS are 2 years and 10 months apart and it works great for us, but now that he just turned 4 I can see that her being closer to 4 before #3 arrived would be okay too... it is so hard to decide b/c you don't know what the personality of the new babe will be, there is no way to predict how they will all interact, etc. I feel like it was always a given that we would have 2 kids, there was no debate, it is practically expected... but 3 seems like a much bigger step.