It really depends on whether or not you are shooting RAW. In RAW, there is not that much difference between the cameras. I do think the newer Sony sensors in the D90 and kx are about one stop better at high iso; however, I can comfortably shoot to iso 3200 with the K7, using noise reduction in post processing.
This is a photo taken at an Aquarium in Gatlinburg with the K7 at iso 3200. No noise reduction done on it, just converted with ACR in Photoshop Elements. There is definitely noise, but to me it is not annoying noise (although if I wanted to, I could easily remove it as well).