I got the Pentax K5 and the K10, and made a comparison using same lenses and settings. The Pentax K10 performs very well in good light conditions against the Pentax K5. The most important factor is the quality of the lenses. If you have high quality lenses go for the Pentax K5 if not stick to the K10. You can find the comparison on Flicker under "Pentax k5 vs k10d Grandson vs Grandpa"

For those interested to see photos taken with these cameras using same lenses an camera setups, go to flickr and look in groups for " Pentax k5 vs k10d". Despite it's age the Pentax k10d performs brilliantly against the k5.