The builder's reputation is very important. A lot can be learned by what is not said from previous clients.
Is this someone you can work with? What does the contract say?
Our last experience was great - He wanted to build our dream house and that was his goal. Another builder we considered was really difficult to get in touch with. Even though he was very talented and probably cheaper we decided this was not going to work. Another cheaper guy who did great work had obvious financial issues - wanted us to go with his bank and that would not work with us either.

Ditto Archie's comments. I would also contact various building suppliers in your area and see if they have any recommendations on builders for you. A lot of times, they will recommend the ones that pay on time all the time, even in this slow housing mess we're in. Ask for contact information for previous clients (get at least 10 if at all possible). Then call each and every one of them and ask about the builder's attitude during the build, quality of work, timeliness of work (does he ensure the job is completed on time), cost of work (does he come in on budget), and would they use that builder again.

Hope this helps!

Your architect is sure to have recommendations for builder's he's worked with. If you don't have an architect or an existing plan, then it will be pretty difficult to start interviewing builders at this point. All you can do is get a general idea of their build costs and level of quality, and that is NOT enough to be able to settle on a builder unless you also have a realistic budget for your wants. And it's hard to develop that budget without some form of house plans.

So, first, decide on how much you can spend, and see what that has built in your area. Bear in mind that a custom build can easily be 2x as costly as a tract builder's subdivision or even an existing home. Building is also not for the faint of heart. It's a long decision intensive stressful situation to be in, and you need to decide if it's for you.