There are a number of different online mortgage calculators available. A very straightforward one is the Yahoo mortgage calculator. All you need to input is your loan balance, interest rate, and term. However, this may not completely reflect your expected payments, especially if you are going to be paying escrowed insurance and taxes and/or private mortgage insurance or mortgage insurance premium.
I use this calculator: http://www.realestateabc.com/calculators/PITI.htm .. it doesn't seem to include PMI though, if that's something you will have in your loan.
dont forget monthly HOA dues (if applicable). like taxes, they account for a significant portion of your monthly payment
It looks like 30 year Fixed FHA rates are at 4.50% right now. They were at 4.25%.
Since you are buying in rural North Dakota using USDA financing, your payment should only be made up of:
Principal & Interest - which you can use any of the calculators the previous posters to calculate
Homeowners insurance - you should contact insurance agents to get quotes for this, start with your auto insurance provider as they may be able to offer you a multiple policy discount
Property taxes - you can find out current property taxes by contacting the county assessor/treasurer's office where the property is located. They may also have the information available on their website, but something tells me most county's in North Dakota don't have full service websites. Just call up the assessor or treasurer, give them the property address, and ask them how much taxes are. Now you should also ask if you buy a certain home for X sales price, will it affect the property taxes? Property taxes often change whenever a home is purchased/sold, so in order for you to budget properly you definitely need to know if they'll change after you buy.
USDA doesn't have monthly mortgage insurance, and I wouldn't think there would be any homeowners association fees.