Definitely the D90. The 50D costs about the same, and it has better FPS rate than the D90, but D90 has movie. Decide carefully. It'll be up to you to choose your preference, whether you want the video or the more serious shooting.

You can single out the 500D, it's an entry-level 1000D with video, that's all. Just a few added things here and there.

Not exactly the same. Quote I Love Money, an upgraded Digic IV processing engine, a sleeker body, 'HD' video. As far as I know HD is 1280x760 30fps. It's not close to that. So yeah. It's just 'video'. HD merely a marketing term.

2. Both results turn out to be very close. In DPReview there is a section in both the D90 and 50D review that shows the ISO comparison. It's marginal. Don't need to worry about that.

Also, although the sensor is produced by Sony, the algorithms and the processing is totally different and designed by Nikon. That's the main point. Yes sensor does play a big role, but the processing engine bigger. Each manufacturer developed their own processing algorithms, and that's what makes the difference.