> I think it would be better if I applied all your principles to the D50 until I know what I am doing before moving upward.
You are wise to recognize this - more people should! It's also worth observing that the more advanced cameras are a mixed blessing, just as a truly great audio system is. With an indifferent stereo, one often does not hear the subtle emotional and spatial clues that really make a recording come alive. However, the best gear - which does do that - also typically has the property of being ruthlessly revealing of a mediocre to bad recording! Same with the more powerful cameras. Sure, you have (for example) a better AF system with which to get 98% in focus at a Formula One race instead of 85%. But 12mp (to say nothing of 24mp or the 60mp from a Hasselblad) will reveal the shortcomings in lenses and especially technique just as well as it reveals additional fine detail. I had a significant adjustment to improve my technique when I moved from the D2h to the D2x, and I'm still going through that with the transition from 400 to 800mm focal length.
> After the Train shoot I tried half a dozen hand held shots of buses but from my vantage point the highlights were blown out where the sun caught bright surfaces.
Not every shot can be accomplished with technical perfection, and in fact some should not be. Just be aware of what's going on and work consciously, and you'll be fine.
> ... 18-55mm lens. Distance from the action is not a problem as I can stand 3 feet away from the athletes.
A perfectly reasonable plan. However, note that the closer you are to moving subjects, the greater their apparent motion is (consider how the view of the ground changes from an airplane a 30,000 feet - perceptively this is much less motion than driving on thr motorway at a tenth the speed). By moving closer you are exchanging a magnification problem (200 or 300mm) for a technique challenge (panning at higher angular velocity). You will also notice that by shooting from in close you will be working with an extremely different perspective. The resulting shots will be very different if they are executed with equal skill. This is particularly true at say, 20mm compared to your previous 150mm. Both are entirely valid choices and I encourage you to try them both to compare the results.
> I notice that when I use Auto ISO the D50 shows ISO between the 4 availabe to the user and goes up to 6400 which is in red.
I'll advise you to read the manual carefully on this particular feature. The implementations have changed over the years in the various cameras and I don't know precisely how the D50's auto ISO works. But in general you have three parts to set: a base ISO (which I'd set to 200), a maximum shutter speed (ie the slowest one you want to use, and id the shutter speed goes longer than this you want the camera to turn up the ISO until it is achievable) and a maximum ISO, beyond which you don't want to go even if the light isn't sufficient. The maximum shutter speed should be set depending on your shooting context - if you're shooting football probably 1/250th is as low as you dare, but for family gatherings 1/60th is usually reasonable. And I turn this off if I'm working in a slow-paced environment and handle the ISO myself. (Actually I personally do that nearly all the time, leaving auto ISO for special circumstances where the light is changing both rapidly and widely.)
> I seldom print out
If your images are bound for electronic display, the higher resolution cameras won't buy you anything! Even on my 27" display I can only show 2560 x 1440 - that is 3.6 megapixels. Anything more and you'll be throwing resolution out to get the image to fit on the screen. At 1280 x 1024 you have only 1.2mp. Very, very few users are equipped to display more than about 1900 x 1200 or 2.2mp. Of course, your D50 is 6mp, so you already have vastly more pixels than you need for electronic display.
Printing is a different story. A printer is usually 240 dots per inch, while your screen is 75 dpi. At 240 dpi, your 6mp file makes an 8x12" print before you have to resort to software techniques. I make 16x20" prints pretty routinely, and 24x36" is not unheard of. To do these one has to interpolate in software, and of course that's another magnification problem: a 36" print at 240 dpi means you need roughly 6000 x 8000, four times what a 12mp sensor produces. Believe it or not, that's fairly straightforward to produce as long as the capture and processing were of high quality.
> optimum shooting size for this screen area and resolutions.
Per above, anything will work fine for that purpose! However, my advice is to simply capture all of the pixels. You usually don't know in advance when you're going to get a winner, and it's far easier to toss them later than to try to synthesize something you didn't capture in the first plsce. Besides, these days CF cards and hard disks are not expensive. Tsking an 8GB card into the field costs just a few quid and a 2TB disk isn't that much more. To put the hard disk in perspective, I have 115,000 images on file and that's consuming about 800GB. Two 1TB hard disks run about $220 these days, so it really isn't expensive. Why two? Because you need more than one copy in the event of failure, accident or theft. (I keep four copies - two on my main system, one on another server, and one off-site 120 miles away.)
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!