Well, it depends on you as the user. Are you already using an SLR, so you know what you need or don't need? You have got to set your own criterias. Do up a checklist of what you want first. Do research on the web to compare the models. Then go to the shops to try out the different models. See which one you like to use. Finally, work out your budget. If not enough money, then don't bother getting the high end ones, cause even beginner models will allow you to practise and get the photographs.
If you are a beginner, the Nikon D40 is an affordable, easy to use camera to consider.