I would strongly recommend waiting as much as possible. I have a very spirited 4-year-old, and when I transitioned him to a big bed at 2 1/2 all sleep habits went out the window. I am serious. I counted up to 90 times that he got out of bed 1 night! And naps became inconsistent from then on. With these wonderful, spirited kids, I think they need as much routine and structure as possible - including a nice solid crib! The new baby won't need the crib for the first few months, and I would wait as long as possible if I were in your position today!
As an aside, my dd is 22 months, and while she is better than my 4-year-old, I am still hoping to keep her in the crib til 3 based on my past experience!