Possible K-5 Purchase
Well, you may get better going Craigslist for the K20, I did, but that was a while ago. As to the value of the upgrade, it's night and day...
I bought the K-5 with the idea of using the K20 as a backup or secondary camera. As one of the early adopters I was, of course, bit by the sensor stain issue. Eventually, I brought my K-5 to Pentax (close by for me in Canada), but was informed it was being shipped to Japan. That left me with, of course, the K20 as my main camera again for three or four weeks. I couldn't do it.
I actually went out and bought a second K-5 because of the difference. It was, literally, a case of a few shots on the K20 and I was going nuts, it just didn't perform, and I totally loved using the K20 before. Net effect, I now have two K-5 bodies, I use them both (handy to have wide and telephoto without a lens swap), and I have zero regrets.
And, yeah, with a good lens you can keep up: