Dude draw the line in the sand right now. Just be honest and tell her that you think the best thing for you is to try and save your marriage. Then do everything you can to save it. Pray, get good counseling and advice, everything you can. Tell bar chick straight up that if it doesn't work out, you would be happy to take her and show her your gun at that point but not before. If she has any class, she will appreciate your honesty and your stock will rise like a rocket....you will be invincible in her eyes for two reasons: 1. You are dead nuts honest, and 2. You are serious about commitment. She will be wet 24/7 just thinking about it.
If you do the right thing, and your marriage works out, then life is GOOD. If it doesn't, and you did all you could, your guilt feelings about it will be severely limited and by that time the bar chick will have thought about getting with you for so long that when you CAN give her the green light she'll be on you like rioters on Reginald Denny. At that point, I would advise her that as her boss you would love to jump in but you can't until she finds a different (read higher paying) job. And she will. Life is then GOOD! And finally, if she says f*** you then....you definately don't need to be involved with her low-class ass anyway. Life is still GOOD.