I would call United Way. They will be able to help you out:
http://www.unitedwaytwincities.org/communityinfo/211.cfm
I'd also contact MN's Dept. of Human Services. They would be the ones to coordinate any economic assistance you might be eligible for from the county and state (Medical Assistance, MFIP, food stamps, and so on).
Can your husband withhold money from you? Until you have an attorney to fight for you in court, he can really do whatever he wants. I would not delay in contacting an attorney. They would be able to help you get set up financially and make sure things are "fair". An attorney would also be able to tell you the ramifications of going through a legal separation first, and the impact that one of you moving out could have on the divorce proceedings. (I have heard that one partner moving out too soon in the process can be contrued as "abandonment" and can affect custody of the kids and so on. I'm not a lawyer, but that's what I've heard. I don't think either of you should officially move out until you know the consequences.)
I don't know how long you've been married, but judging from your post, it sounds like you will get alimony, at least for awhile. But you also need to think about getting back into the workforce. The company I work for right now is hiring. We do non-medical, in-home care for senior citizens: light housekeeping, errands, meal prep. We do NOT do anything medical so you do not need any special training or certification. The hours are very flexible and they are hiring for positions in St. Paul and Mpls right now:
http://minneapolis.craigslist.org/etc/1130909598.html
I would end your online relationship. If your husband catches wind of this, he could use it against you in court. Even if you are deleting the history on the computer each day, he could get a court order to have the computer confiscated and examined, and stuff you deleted 2 years ago could still be on there. Also, unless you have actually met this guy in real life, don't take for granted that he's "real". To be blunt, you are in a very vulnerable position right now (financially, emotionally) and con artists can smell that a mile away. It could be a 15 year old boy who posted his dad's photo and is getting the biggest kick out of this all. I have known more than one person who created a fake online persona to be "funny". Even if you HAVE met him, what kind of guy goes after an unhappily married woman?? PLEASE be careful. You also have your kids to think about, they really don't need any extra drama.
Good luck to you.