On my website, there are 2 calculators you can use that really help you gauge what the monthlies will be.. Look on the "Figure out the Money" page. Here is the link also.........
What you should do is generate the info off the calculators for different scenarios, and have your computer with this page open when talking to a mortgage broker...this way you can generate different scenarios depending on the interest rates, points, etc. they are quoting you.
Base your decisions on a fixed interest rate of 30 yr basis. Have this common denominator of numbers printed out. Then go from there. Remember to include your monthly costs of taxes and insurance, HOA, (home owners association) fees, etc. also!