Professionals can take high-quality published images with a point and shoot. Plenty of folks shoot junk with 1Ds MkIII's. I'd suggest starting with a mid-level DSLR, more than capable of superb images, and spend $$$ on trianing courses, shooting trips, etc. The only limitation is if you're truly looking to shoot "professional" as in selling images to stock agencies for resale. These agencies have technical limitation that you would need to look at.

As mentioned above, you need to budget for more than the body. $1800 will get you a D300, the best in that price range, but if that's at the expense of limited spending on lenses, it's not a good investment.

Personally, I'd stick with Nikon or Canon, given the vast array of lenses, accessories, other shooters you can get advice from, etc. Sony/Olympus/etc make competitive gear in certain price ranges, but nothing I see that stands out to the extent it overshadows the above.

At the D60/D80/XSi/40D range, it comes down to personal preference. Both companies will provide strong offerings for a long time to come, IMHO. they've leap-frogged each other over time, and will continue to do so, but the gaps are generally small. Put your hands on all of them, work the controls, shoot some images, and see what you prefer. I don't think you can go wrong with any of them.

I've been invested in Canon for a long time, and the XSi (and the whole Rebel range over time) is too small for my hands, feeling like a bit of a toy and hard to work the controls. I also couldn't live without the control dial on the rear, so I'm shooting with a 40D.

From what I've read, the 300D is the best buy (and only option) at the $1700-1800 range, the 40D is better than the D200 in the $1100-$1400 range, and I can't really come to a consensus at the <$1K range.