Michael, I also have been looking for something in the 400-500mm range, I find that I must crop the images from my 300mm to much. I also have not found anything that my wife thinks that I can afford, being retired. The 70-300 is great for the grandkids soccer games but they mostly play in the evenings and the lens is a little to slow and the K2000 high ISO is not to good. So I have also been thinking of a 12mp sensor like a Kx or Kr, better ISO and better results after cropping. My 15mp point & shoot crops better than my K2000, are you happy with 50% crops out of your Kx.
...It is better to burn a roll of film than curse the darkness. Equip. K30, Q7
, DA PLM 55-300, DA 18-135, DA 35 f2.4, DA 50 f1.8, SMC-M 28 f3.5, SMC M 50 f1.4, Canon P&S S100 w/CHDK Beta, Panasonic DMC-GM5, Flickr: