If you worry is that the value of your townhouse is lower than it was a few years ago you need to consider that the value of the larger home that you want to buy is less as well. As a result it may be a "wash" when it comes to the money that you need to make the move to a larger home.
The best thing to do is contact a local realtor who can give you a more exact answer based on the value of your current home, what it might bring for rent and what your new home might cost. You may also want to talk to a mortgage professional to see if keeping both homes is financially feasible for you.
Good luck and if you need assistance feel free to contact me.