Turbo does use more power than the same CPU would without turbo. It does not exceed the rated TDP, but that doesn't mean that power draw is not increased. It also gives a significant performance advantage though. Personally, I'd get the i5.
Oh and turbo can run with 2 cores active. A friend of mine recently purchased a computer with an i5-430m, and even under 4 threaded benchmarks, it held 2.53GHz most of the time.