B.D.
Wow, you sound like us. We have a boy and a girl, but both of us feel like we could have another. We also struggle with the sleepless nights, expense, jobs...well you know the list. We recently decieded, after much of going back and forth that if we were going to have another kid, we would try till October and if we didn't get pregnant it wasn't meant to be. After October we are going to sell all our "baby" stuff to make sure we are serious about our decision.
As far as bringing it up to your husband, ask him how he feels about another one. I would do this when the kids are in bed and you can have a one on one talk. This conversation doesn't have to be the end all of the discussion either. My hubby and I have been talking about it for at least 4 months!! We just random bring it up when we have time together to see if either of us have any new feelings or ideas.
Good luck with it...I know exactly what you are going through!!