A couple of thoughts from a lending perspective:
1.) Only 75% of your gross rental income can be used towards qualifying income. This means if you charge $1000 a month for rent, with an $800 a month mortgage payment (including taxes, insurance, and HOA fees), you will have a negative cash flow for qualifying purposes of $1000x75%-$800= (-$50.00). This effectively reduces your qualifying income.
2.) You must have a fully executed lease AND cancelled check from the tenant for security deposit or 1st month's rent in hand to use any of the rental income to offset your mortgage (for qualifying purposes)
3.) Fannie Mae and Freddie Mac have issued guideline chnages that significantly tighten the qualifying standards for income producing properties especially when converting a primary residence to a rental. If you cannot prove that you have at least 30% equity in your current home (using a BPO from your Realtor or an AVM from your lender), NONE of the rental income may be used to offset the mortgage expense of your current residence for income qualifying. This means you have to document enough income to pay both mortgages plus all other debts.
4.) Reserves: You must prove 6 months' worth of mortgage payment, taxes, insurance, and HOA fees in a bank account for BOTH properties if you cannot prove 30% equity in the residence you are converting to a rental property. If the equity is there, you could get away with just 2 months' reserves for both properties if Fannie's automated underwriting software says so.
The changes above apply to loan applications dated AUgust 1, 2008 or later. Some lenders may apply them in advance of that date.
If you are unable to meet the new guidelines, your loan is inelgible for delivery to Fannie Mae. I've heard that Freddie Mac will soon (if not already) adopt the same changes. Fannie and Freddie are really hitting investment property owners hard on qualifying.
Since you already have a Pre-Approval, you must make your decision very soon. You'll have precious little time to locate a property, negotiate a sale, and submit your application prior to August 1st.