You need to know some things about obtaining financing with the townhome (if it's not paid off). I will assume it is not paid off.
You will need to qualify for the new property using your current income against your current debts that are found on your credit report. You will also need to add to your debts, the new house payment and your old house payment. If you divide your monthly income by your total debts, your percentage cannot exceed 41% for Conventional loans and 55% for FHA.
If you already have an FHA loan on your current home, you cannot obtain a second FHA loan (except for very few situations). Conventional programs will not allow you to use the rental income on your town home to offset the mortgage payment unless you have been renting the home for at least 1 year AND the home has at least 30% equity in it. FHA will allow you to use the rental income against the mortgage payment if you have been renting it for a year. The 30% equity rule does not apply to FHA guidelines.
Once that's cleared, make sure that you can afford to make the mortgage payment on the town home in case your renter skips out on you and you have to make the monthly payment. It's best to find a management company to rent your property. This avoids you the hassle of collecting rent, evicting a resident etc.
If you find a lender that tells you that they can use the rental income against your townhome mortgage payment, be prepared to testify in court. Just kidding. The guidelines changed over the last few years and few Loan Officers read guidelines.