ISO confused

...so therefore I am experimenting. With the new cam. First up: ISO settings.

I understand the concept of the different ISO settings (if you can remember back to non-digital camera days, it's the 'speed' of your film. Basically it is how sensitive the film is, higher the number = higher sensitivity. And if I remember correctly most common was 400, lower numbers for outdoor shots, higher for low lighting).

But knowing which setting to use and when is another story. Since I am a visual person, I'll share my results because I know you must really care, right?

ISO 100

ISO auto

ISO 1600

This stuff is probably obvious to everyone else but the last time I used manual settings on a camera was ages ago. And the last time I used an SLR was eons ago. By ages and eons, I mean since high school (which is the same thing). And even then, I didn't pay attention. Because I don't do numbers very well. I mean even the lowest ISO setting is 10 times higher than the amount of fingers I have (at least that's what my calculator says).

Will I master the prosumer dSLR? Or am I only suited for point-and-shoot/auto mode forever?