@Nicole:
@Jdepould: Thanks for the tip; in fact, I was studying the Tamron 70-300mm last night. For a very inexpensive, amateur piece of glass, it appears to be a contender for the subject Pentax lens. I basically want an outdoor/day lens with some reach that I can use with a relatively fast shutter speed (1/250 to 1/500) without having any major softness or exposure issues.