Regarding noise... as Na Horuk says, you'll get
some noise at ISO 800 on the K-x. Even later models will start to show a bit of noise at this sensitivity if you look for it, or pixel-peep.
Assuming you're not using a preset or other plug-in for the black & white conversion, and you're just selecting "Black & White" under the Basic section of the Develop module, I would recommend setting your noise reduction as follows:
Luminance 25 (or less, to your personal taste), Detail 50, Contrast 0
Color 25, Detail 50, Smoothness 50
More noise reduction than this will likely soften detail. The luminance figure should be set at the minimum value you're happy with for the intended reproduction size. You're not looking for all noise to be removed at 100% (i.e. pixel level) viewing, but for an acceptable amount at the required viewing size.
Some noise is absolutely fine - after all, you'd see that with film too. You're only looking to manage it so that it's not too prevalent.
Sharpening can easily accentuate the noise, so don't over-sharpen and always use the masking slider to minimise sharpening on everything except edge detail. I'd recommend starting with the following settings:
Amount 25 to 50 (to your preference), Radius 0.7, Detail 25, Masking 80
Hope this helps