It looks like you are going into DSLRs. Does that mean you don't want consumers or prosumers (which cost less, easier to use, but have inferior capabilities)? Are you experienced enough to take basic pics with a DSLR? By going into DSLRs, I am also assuming that size and money isn't a concern to you (DSLRs lack in both respects).
I don't know much about DSLRs so it's hard for me to say anything. The Canon 300D is considered entry-level DSLR, while Nikon D70/Canon 20D/Pentax *istDS/etc are considered mid-level. THe prices and features reflect these two levels so it comes down to what you want and how much you are willing to pay. Most people pick a DSLR based on the lens system that they would want to go into. Typically, throughout a DSLR's life, you would spend more on the lens than the DSLR. Depending on your budget, I would buy the best lens you can afford (and need) and then buy the DSLR that fits that lens (and your budget).