Jeanette offered some good advice, I'll add some suggestions:
1. Talk with a knowledgeable Realtor about the local market conditions. You need to find out a few key statistics:
A. How many homes, with the same characteristics as your present home, are on the market now? They will be your competition. How long have they been on the market?
B. Compute the absorption rate. Ask your Realtor. It's the total number of homes on the market, divided by the number of homes sold last month. The rule of thumb is that around six months is "average market", less than that probably means a seller's market, more than that probably a buyer's market.
C. Figure out how much you want to net after you sell your current home.
D. Talk with your Realtor to determine if your net goal is reasonable. Then talk with a lender about what financing options you have in order to buy your new home.
What the new construction builder will do is based on the market conditions. If he has a lot of new homes sitting around vacant, he may be offering special incentives to attract buyers. If your market is a seller's market he may not be cooperating with brokers and basically holds a gun to buyer's heads and says, "here, I dare you to make an offer".
If the market is "slow", the builder might accept an offer contingent on you selling your home. Most builder's require that you have your home listed with a Realtor in order to accept your contingent offer.
Renee, I hope that your market is "average". Get the facts, then act quickly. Summertime is the time most home sales take place (before school starts), so all things being equal, now is the time to move.