I have found that with my camera (Nikon D700) it just goes down hill quickly after ISO 1600, dynamic range is reduced, becomes noisier but doesn't actually improve the image itself. Sometimes what shows on the back of the camera is not a good indication of what is actually in the image. I have found that at ISO 800&1600 my camera (30s images with a 24-120mm @35mm @F/4) do not look crash hot but far better after some processing. This in comparison to ISO6400 images which look great on the back of the camera but not so much when processing.
I have found that a good website is
http://www.sensorgen.info/ , it may not always been 100% accurate with results but they're close enough to give an indication.
Your camera,
http://www.sensorgen.info/PentaxKM.html
It shows that after ISO 800 you do not actually get an improvement in image quality, actually gets worse.