One thing you have not mentioned, that needs to be addressed! Have you ever thought of an AIDS/HIV test for your husband, or yourself? Obviously he had unprotected sex with this woman, and that should be something that should be addressed. He has made the possibilities of both of you with a truly deadly disease!
The next thing you need to think about is how you are going to handle having your husband with another person's child?
If the two of you do stay together, is this going to be something that you are going to throw in his face, every time money goes out of your house for doctor bills, clothing, school expenses, and such?
I think both of you need to sit down, "alone" when children, phone calls, and such can not disturb you, and attempt to talk out all the variables that may pop up. To have rules and such so you don't wind up making both of you miserable down the road with his transgressions.
Who knows, best scenario the child is not his, and you can move on, but if it is, like the other poster has said, his responsibility ends with his child. It may not be as easy as it sounds, because she has two other children to deal with besides your husband's child, and her life may be less than what your husband would be willing for his child to live in. Then, could you handle having that child living with you on a permanent basis, facing you each day with his infidelity? This could happen, and you need to address that possibility as well. Life is not promised to us, and she could wind up in a situation where she could no longer take care of her children, and you may just wind up with this child living with you.
Again, I think you both need to sit down and discuss all scenarios that could happen, and how you are both going to be willing to handle them. Then, when you think you have covered all possibilities, something else, not thought of will pop up.
I hope this helps, good luck, and please keep us posted. I am sure your situation will help many others.