If your new to photography, I'd really suggest you go down to your local dedicated camera store (preferably not a Best Buy/Future Shop), and talk to the people there, all the while getting a feel for the cameras.
When it comes down to it, each camera has their own pro's and cons, but nothing beats physically handling and trying out all of your options, especially when working with SLR's. The weight of the camera in your hands (along with added battery packs), whether the body feels "cheap", and the lenses offered all can play a big role in what you decide to go with. For example, while a friend of mine prefered the usuability of the Canons, and even already had an old Canon lens she acquired, she ended up going for a Nikon (it might've been a D80, I'd have to ask her), as she felt it had a lot more weight, and would handle better in the environement she'd be using it in (sports photography, particularly swimming).
Like makaroni said, it's an excellent idea to inform yourself as much as possible. Compare, read reviews, ask staff....it all helps a lot. By no means am I an expert (far from it), but I know from current experience that it's crucial you know exactly what you want to buy. I'm moving from an older Pentax 35mm I got from my Uncle a while ago, when photography was more of just a hobby, to a 30D which I'll be using for a lot of the more serious portfolio work I'd like to do in the coming months.

Last thing I'd like to point out, is that image quality is very dependant on the type of lenses you use. You won't see as much difference in quality with varying megapixels with SLR's, as you will with using cheaper glass over better options. So always make sure you go for the best quality within your range, even if it means getting a less-expensive body. Keep in mind, lenses are interchangeable, so you can always use them with a newer body in the future.