The higher the ISO (sensor sensitivity), the more noise you get. Noise are random little spots. They are the size of a single pixel and there are ways to reduce this noise (which have some downsides as well). Nobody notices a little noise, but it can get overwhelming.
In the film days, it used to be film grain and high ISO film had more coarse grain. But back then, you were stuck with just one ISO per roll. With digital cameras, you can choose between many different ISO values. One other thing is that modern camera sensors are much better than older ones, so they can reach higher ISO values, they have less noise at all ISO, and so on.
So I would suggest you get a used K-r or K-30 or K-50. Even a K-200D will be a better choice than the *ist, simply because every generation digital cameras improve a little. The main thing to keep in mind is that you cannot change the sensors. Digital cameras have one sensor and you are stuck with it, for better or worse.
And newer cameras have other perks, like faster performance, more compatibility with modern gear, and so on
Here is a photo to look at:
http://i.ytimg.com/vi/RIMw3dn6doI/maxresdefault.jpg
(its from
this video)
See, at 100 it is clean, but at higher values, the photo has noise. And that photo is from a pretty good camera, older cameras will be much worse even at ISO 1600.
But what else does ISO do? It lets you take photos in darker places. So if you are indoors or it is getting dark, you will need to raise the ISO (or use flash, tripod.. there are other ways to take photos in darkness, but raising ISO is the easiest). At low ISO, 100 and 200, there won't be a big difference between older and newer cameras in terms of noise.