In response to the original thread: if you are on a budget, consider the K-x or the K-7.
The K-x is smaller in size. Is this important for you ? You can only answer the question yourself.
The K-7 has WR. Again is this important ?
I have a K-7 and I shoot up to ISO3200. I do not shoot above ISO3200, but I do shoot in low light conditions: dusk, dawn, sunset, sunrise and indoor. Be real... no one want to shoot at ISO in purpose unless you want some grainy effect.
So let us talk about low liht conditions. In low light, I simply use a fast lens: i.e., a lens with large aperture and low f. I couple my K-7 with a Voigtlander Nokton 58mm f1.4 with great success. There are other great fast lenses (f1.8 down to f1,.2) with Pentax mount. The fast lens gives you great shots at lower ISO than other lenses. For example, with my Voigtlander 58mm f1.4, I never need to shoot above ISO800 at dusk or dawn
Hope that the comment will help.