J.F.
I am a Realtor in NY, so this may vary a little as you are in TX, but...
The way the market is, I would also recommend you get your house on the market. Now is a good time. By the time you get your land secured (About a months time) and you get an acceptable offer on your home, plus time for them to build, you are looking at 3-5 months from now before you could really move into that new home anyway.
A new home construction loan is done in draws, so you dont actually pay anything on the loan until your house is complete, has passed all inspections and you close on the loan. As far as the land goes, you CAN have it rolled into the new construction loan OR you can buy the land you want before they actually start building. Just keep in mind most people buy land with CASH. (Least they do in NY state)
I hope that helps. Some contractors are willing to buy the land for you (with a signed contract) and will also build the house then "sell it to you" as you would buy any other home on the market. You would again not need to make payments on the new loan until you close on the new house, but if you have not yet sold your existing home you will be making double payments until it sells.
You may also consider a rent to own or seller financing with a certain percentage down IF you are not having luck selling the house.
Good luck:)
J.
www.mygc.com/jfiegl