Bear in mind that each of these SLRs has their own lens mount. This is a deliberate ploy by the manufacturers to lock you into their system. If you already have some Pentax lenses then well worth looking at the *istD and *istDS.
If you haven't already got a load of lenses then check out some of the new superzoom cameras. Due for release this month is the Fujifilm FinePix S9500 (28-300mm equivalent) and next month the Panasonic Lumix DSC-FZ30 (35-420mm equivalent). These have lots of advantages over a DSLR - cheaper, less to carry around, sensor doesn't need cleaning every year, some also have optical image stabilisation.
Another site to wade through is:
http://www.steves-digicams.com/
Check out the 'best cameras'.