Back to the OP:
I have been looking closely at DSLR's for a while. I shoot 35mm and medium format film, and have been patiently waiting for DSLRs to deliver the imagery AND convenience I am used to from film. I am pretty brand-agnostic -- my 35mm cameras include an almost-new Nikon F80, and some classics (Canon TL and Pentax Spotmatic F.)
Having a little bit of experience with SLRs helped me cut through the confusion as I look at DSLRs. Here is what I have learned:
1) They all make great pictures. Don't sweat the image quality between brands or between cameras. Unless you are going to shoot portraits of presidents or landscapes that will be blown up to 10'x30', every DSLR delivers the goods.
2) The viewfinder matters. Put it up to your eye -- can you easily see every corner of the frame? Is the image bright and clear? Can you easily judge focus? This is mostly a personal preference, and the technical specs can't tell you how comfortable the viewfinder will be for you. The viewfinder is (after the lens) the most important part of a camera, because it is where you make every critical decision about framing, composition and focus. When you are photographing, you live in that viewfinder. My Canon TL has the best, brightest viewfinder of them all, the F80 is OK, the Spotmatic is horrible. Yet the Spotmatic F is a very highly regarded classic camera -- so the viewfinder isn't something that most people discuss. In the current range of DLSRs, every camera offers a different viewfinder experience. The best I have used so far was the D70. The D40 and D80 are pretty good, the D50 is good. But your evaluation might be different.
3) The grip matters. Do you like holding the camera? If you do, you will be more likely to carry it with you. Stupid, but true. I dislike the feeling that I may drop a $600 camera at any minute -- and the Canon Rebel and the D40 both give me that feeling. It's a combination of grip design and materials used. In my hands, nothing feels as secure and comfortable as my F80, but the D70 and D80 are very sure-footed. The D50 is quite comfortable. I also really like the grip on the Pentax K100D. Your mileage will vary, so you have to try them out. The importance of the grip increases exponentially when you add a larger/heavier lens. With a kit lens, most cameras will feel OK. Now imagine adding another 1.5 pounds, hanging off the front of the camera. Still feel secure?
4) Control. You need to know/guess/play around with how you will use the camera on a daily basis. If you will be putting the camera into "Full Auto Program" mode, then don't worry about controls. Any design will do, because all you really need is a shutter release. However, the reason I am looking at DSLRs is because I want to have a digital camera that gives me the same control as my SLRs and medium format cameras. Using the "Manual" exposure mode, where you set the aperture and shutter speed yourself, is really only useful if the controls for aperture and shutter are immediately available and readable. The D40 for me is VERY crippled by not having the secondary LCD screen. You have to push the INFO button to get the camera display fired up, and then adjust the shutter speed and aperture using the command wheel and aperture button. Compare this to my F80, or the D70 or the D80. Set the camera to Manual, change the aperture with the wheel under your index finger, change the shutter with the wheel under your thumb, and read both settings off the top LCD screen. It seems like a small difference, but to me the added steps to use Manual control in the D40 will keep me from using Manual controls as much as I like. The D50 is a reasonable compromise, simply requiring you to press the Aperture button to use the single command wheel for both Aperture and Shutter.
The bottom line is: you have to handle all of the cameras. Give yourself a good long time to play with them. Think about it for a day, then go back to the store and play with them again. Give yourself more than a "first impression" of the cameras. See which one is comfortable, lets you visualize the picture you are taking, and lets you shoot how you want to shoot. The nice thing is that there are actually some choices in your price range now. A couple of years ago, we were stuck with just the Digital Rebel. I'm glad for options.
Best of luck! Tell us how you do.