I am hoping to have a K10D to play with soon, I am hoping that my Sigma 24-135mm will work well with it. It is an f2.8-4.5 lens so it is quite fast at the wide end and acceptable at the long end (just over one stop, SR should manage that much). My only experience with SR is to occasionally use my wife's K100D Super (which I try to avoid since I do not want to be spoiled).A K10D, even one with possible focus issues (it is used), will be a great step up from my DS/DL combo in terms of resolution and features. I believe that a different camera body may actually have an impact on which lenses which I use, the DL with fast 50mm will still be the low-light king but a K10D with the Sigma may well become the constant companion camera.
Glad to see so many responses, I found a blog site http://raffee.blogspot.com/
(please excuse the name, not my fault) which is entirely made up of images shot with fast primes, including quite a few with the Pentax 50mm f1.4 and even a few with the old 50mm f1.2. Worth a look since some of the images fit the "look" I referred to at the beginning of this post.