After purpose, I'd put budget as a very close second, though admittedly that can be a bottomless pit, it will dictate what level you want to enter at.
If you're going DSLR , and if you're serious about photography that's the only way to go, decent camera bodies can be had for ~$500 up to thousands of dollars, but a $1500 budget can get you a very decent setup, less if you just want to start w/ one lens to practice and polish your techniques. As you shoot, need will dictate what other lenses you may add later.
Also allow ~20% for ancillaries, that is memory cards, bag(s), tripod/monopod, cleaning cloths, batteries (or an extra one if it's dedicated), etc..
As far as brand, as Keg said any of those 3 he mentioned will do a good job. Other brands are also emerging more and more as Pro style.
I always recommend going to a dedicated photo store as many will give you 10-14 days trial, plus the after purchase help, especially for a beginner, will more than pay for the price differential of buying on-line or at a big box store.
Here are some great sites for product info and reviews posted by Keg in another thread:
http://www.the-digital-picture.com/http://www.dpreview.com/http://www.steves-digicams.com/http://www.dcresource.com/http://www.luminous-landscape.com/Lastly, have fun!
Anything else, let us know.