Bless ya, I can understand your feelings. I am sure your boys will be so excited about the new arrival but I know the novelty won't last and reality will kick in. I'm not really in the same boat so can't offer much advice, but I am sure your boys will surprise you. I don't know how mature they are and how much they will understand your decision but I would say try to involve them as much as possible so they feel a part of it. They may be more understanding than you think, especially if they feel like they've had an input in it.
I don't work (well, a couple of hours a week running a swim school, but not a significant contribution to the household finance), but this was a decision me and my OH took together. We decided that it was more important for me to be home while the kids are young than it was to have big holidays and expensive things. We are lucky that my OH's business does very well so we are still able to have a nice house and 2 cars etc, but the thing that means most to my kids is that I am there to pick them up from school, and play with them and do all the everyday stuff. They do get plenty of things but even if we couldn't afford it all, I still believe that they would be just as happy.
By the way, your willpower is amazing! My kids knew I was pregnant as soon as we did and all our friends and family knew within a few hours! We were just so shocked and then excited!
Hope you work your feelings out and good luck for telling the boys. I know there are ladies on here who have older kids so hopefully they may post too with their experiences of how their kids reacted