During my IT career I did a lot of interviewing covering all grades from Trainee Programmer to Project Manager. I would recommend the confident approach every time. It's possible to convey a confident attitude without being arrogant, boastful etc. It's very simple. If you don't appear to be confident about yourself, why should the new employer think otherwise?
The old rules still apply, maintaining eye-contact, speaking clearly, neat appearance, listening to questions before replying in a concise and informative manner, demonstrating that you know what you want from the job on offer and your career aspirations. If you don't think you have understood a question, ask that it be repeated or clarified. If you don't know the answer to a question, don't attempt to produce a BS answer -it'll be spotted! An indication that you've taken the time to learn something about the prospective employer's industry/company will impress. Finally, don't worry if you are nervous while you are settling down. It's perfectly normal and it's allowed-for.