The K-x with the kit DAL 18-55 is capable of taking very crisp, sharp shots.
Flickr and these forums are full of sharp K-x images mostly using that combo.
The enemies of sharpness are legion, but include motion blur caused by camera or subject movement, missed focus, and lens quality.
To improve your results, a few suggestions:
- don't use a UV or protection filter on your lens at all - many are garbage and can turn your images into mush and impede correct AF function. Use the supplied hood instead;
- choose a tighter AF cluster than the default Auto 11. Use Auto 5 or Spot AF (p-117 of the K-x manual) in order to get more control over what the camera may choose to focus on;
- make sure your Custom Image settings are set to something like Natural or Bright (the default), with default settings for sharpening;
- this may sound obvious but make sure you are choosing the right shutter speed settings for the scene. Usually P mode or the other auto modes get it right but if in doubt go to Tv mode and set a decent shutter speed yourself. People shots in particular can be demanding;
- set the Auto ISO range in the K-x to allow a decent maximum ISO - eg at least 1600 ISO, or even experiment with higher ISO's. Bumping up the ISO ceiling may allow the K-x to make some better decisions about what shutter speed and aperture it can use in some of the auto modes. The K-x has considerably more ISO headroom than the istDL, without any major image quality penalty until you get over 1600 ISO.
While you can hack the K-x to allow AF fine tuning via debug mode, I wouldn't bother with that.