The most important thing to remember if you are buying a DSLR camera is that your are really buying into a lens system - the body can and almost often is changed at some point, but you keep your lenses.
So, you need to look at range and price of lenses and read plenty of reviews while deciding on body and lens combination.
I personally would not recommend the Pentax given the reviews I have read, but that is just my opinion.
Between Canon and Nikon, both are very good cameras, and it depends how much into photo editing you want to do as Nikon tend to give sharper pics out of camera, where Canon are softer pics, which IMHO is better, because you have more control over the sharpening out of camera.
I have owned a 300D and now have the 20D - I chose Canon because of their L range lenses and their compatible lens range which are IMHO hard to beat, although I do not own an L lens yet because they are expensive! I also prefer the look of Canon pics straight out of camera.
The Nikon D70 and the Rebel XT are sort of equivalent give or take a few settings. If you want to take the kind of pictures you are taking, then I would probably opt for either the D70 or the XT - but take a peek at the 20D if you can afford it because it beats them both on settings, which is important for the kind of photography you are doing.
However, only you can decide. remember the lens system factor when buying and also I would suggest reading some reviews on both cameras and even the Pentax.
Read reviews focusing on the type of photography you are aiming for, and with either camera, you will have to buy a new lens as they are only small zooms, and I would imagine zoom is important.
You will need a decent lens for the type of photography you are taking, and you need to read plenty of reviews on lenses, since you need a long zoom lens that can cope with a fast shutter speed at full zoom and small aperture sizes, although there is always compromise when you alter aperture/shutter speed, it is finding the right balance.
Visit
DP Review for their excellent guides on lenses and cameras and they also have forums there where there are people always willing to help you out. You can also view people's pictures so it will give you an idea of what lens + camera combination is capable of.
You really need to do a lot of research so that your money is not wasted, and whatever DSLR you get, you will need to do some photo-editing. this may seem a laborious task, but you can do your own workflows to save you a lot of time!
I hope this helps!
