The low end Nikon dLSRs (D40, D40X & D60) have slow autofocus (comparitively) and a limited selection of lenses (again, comparitively).
Among the cameras you mentioned, the Canon XTi has the fastest autofocus and it has a 10MP image sensor, like the D40X & D60, and unlike the XT (8MP) and D40 (6MP).
The Sony, Pentax and most of the Olympus dLSRs have sensor shift image stabilization in the camera bodies, while Canon and Nikon use optical image stabilization in certain lenses, making them bigger, heavier and more expensive. Image stabilization reduces (if not eliminates) motion blur due to camera shake.
But Sony, Pentax and Olympus have smaller selections of lenses than Canon and Nikon (except the D40, D40X & D60. These camera bodies don't have a built-in autofocus motor, so most lenses for Nikon cameras won't autofocus on these cameras.)
You haven't said what kind(s) of photography you like or want to do, and different brands and different models may be well suited for some things and not so good for others. Perhaps you could tell us how you plan to use your next camera.