It's worthwhile pointing out that K-Ar dating isn't used a lot any more. It is used, because it's pretty low-cost and is understood very well, but it is seldom used as the sole determination of an age. And, of course, it is beloved of creationists because they understand it and there are some real potential problems.
Do I surmise correctly that you don't need to know the propensity of Rubidium and Strontium to ihabit the different minerals, but merely isolate separate crystals from the rock and measure the abundances of each element?
Yes, with one minor correction ... what you refer to is called a "mineral isochron". There is also such a thing as a "whole rock isochron" in which each sample is a rock from the same source, but each rock/sample consists of many different minerals.
What is actually required is a set of "co-genetic" (that is, from the same source) samples in which the ratio of
87Rb (the parent isotope) to
86Sr (the non-radiogenic isotope of the radiogenic daughter isotope) differ. This allows the isochron method to not require the assumption of zero-initial-daughter and also allows it to almost always indicate when the assumption of a "closed system" has been violated or when the samples are not actually co-genetic. A terrific example of the power of the method
Precise dating of the destruction of Pompeii proves argon-argon method can reliably date rocks as young as 2,000 years, in which the Ar-Ar isochron method was used to date rocks of well-known age. There was "excess argon" (the technical term for argon being incorporated into the rock at solidification) present, but the method worked anyway.
The basic isochron method is discussed in significantly more detail at
Isochron Dating. Isochron methods are "age-diagnostic" methods; they not only give us an age, they give an indication of how likely it is that the age is correct. The vast majority of dates analyzed today and recently are done with age-diagnostic methods.
The other most commonly used method, probably the most commonly used method, is Uranium-Lead concordia-discordia dating. I don't know of any good explanations of that on the Web, but I sketched one out in
this mewsage.
The canonical non-specialist reference is "The Age of the Earth", G. Brent Dalrymple, Stanford University Press, 1991. There are some useful collections of links at
Radioisotope dating links and information and
A Radiometric Dating Resource List.