Post processing sure can make a difference on the final image. Photo #2 above is heavily processed. Has little to do with the camera because anyone can process to this effect with the know-how of post. You can tell by the light illumination by the rocks on the left and right as well as by the color of the reflection of the water.
#1 and 3 are also processed though a little less so.
iso = international standardization organization. This was the "film speed" when film was used. It's a term still used and is best explained like this.
ISO 100 = 100 worker bees gathering light.
ISO 200 = 200 worker bees gathering light and so you need less light than iso 100 to have the same exposure.
ISO 6400 = 6400 worker bees gathering light and you need even less light to gather enough data for the image to be made.
So in very non technical terms, use lower ISO, 100-200-320-400 when there is abundant light. Use higher ISO when there is a lot less light. I can't explain how much is enough because every camera/lens is difference and it takes practice. But that's also the beauty of the LCD on the back. You can correct things right away.
You should likely buy or look online for some basic photography tutorials. Also this place CAN be a decent place to start if you can get passed the retarded gearheads and fanboys.
http://forums.dpreview.com/forums/forum.asp?forum=1002Btw, the reason your shot looks so ho-hum is because you are shooting a landscape with a very bright sky and a very neutral-dark foreground. The camera tries to evaluate the entire photo and pick a happy medium for exposure. The result is less than flattering. The result is what you posted. There are two ways to get around this.
Before digital one used a graduated neutral density filter - darker on top and lightens toward the bottom. This made the sky retain its texture and detail while the foreground remained the same. This was fine. Today, many people stick their camera on a tripod (a sturdy one is best) and they take 3-7 exposures at different exposure values from dark - where the sky would look best, to very light where the foreground is exposed best. They then combine said images using software and the result is a sky that is exposed properly and a foreground that is exposed properly as well.
There's much more to explain but I think a book or some online reading would be best.