This is just what I've found ...
A quality 70-200 f2.8 seems like the magic jump off point after which size, weight, and $$$ go through the roof into professional territory and really require monopod mounted shooting. Part of this effect is who the camera companies are marketing to, and the other part simple physical characteristics ... once you go past 200mm and f2.8, the physical size of the glass elements just gets huge, and that costs money. The Nikon 70-200 F2.8 with vibration reduction is $1,500. An astounding lens with rave reviews, but expensive. The Sigma 70-200 F2.8 (no VR) is $800 (which is what I use). I got my Sigma on eBay for $600 basically new.
An example of the 200mm/f2.8 jump off point: The only 300mm reach zoom at F2.8 that I could find was the Sigma at $2,000 and its 4.5" diameter, 10.5" long, and weights 5.7 lbs. You'd better be in good shape if you're going hand-held.
There are 100-300mm zooms that are fixed F4.0 for slightly more than the 70-200 ... but my approach was to go with the 70-200 at F2.8, then add a matching Sigma HX extender when I need the extra reach, which put me at 400mm and a fixed f5.6 (a 1.5x extender would put you at 300mm and F4.0). This way I got the best of both worlds ... a big aperature at a manageable size and budget for infield or indoors, and extra reach when I needed it at the same aperature as the longer fixed lens. My big fat 70-200 is actually faster and better indoors in low light than my little 28-105, which I just use for casual stuff. If you use an extender, make sure it's a high quality one that matches the lens. Any extender will affect quality somewhat, but cheap extenders ruin what you're trying to accomplish with a good lens. The 2x extender I use primarily for s o c c e r (daughter) where the shot I want can move from 50" to 250" away very quickly.
The sunset I posted previously was shot with my 70-200 ... hand held, holding my breath, in between heartbeats
. I was at our dinner table at a restaurant in Maui and the wife made me leave behind the tripod ... you know, family dinner and all. But I did sneak in the camera
. Being able to shoot at f2.8 made that shot possible. The compressed depth of field at 200mm (effective 300mm on the D70) also made for a nice shot. The distant clouds seem like they're almost on top of the sailboat.
Is the Vibration Reduction of the Nikon lens necessary? That's a beautiful lens I'd love to trade up to someday, and the VR would allow me to shoot in lower light, slower shutter speeds, e.g. night games, without having to crank up the ISO ... in the mean time I either use a mono-pod or just very good shooting posture. We have very few night games, so it hasn't been an issue.
I can't really say which body to buy, D70 vs 20D ... they will both take beautiful pictures. I'd tend to pick the lens I really wanted for the type of sport I was going to be shooting, then figure out what I could afford to spend on the body. Both these bodies can do up to 1/8000th which can give you some spectacular freeze action, but you just can't get to those speeds with a low ISO if you can't let enough light in through the lens.
