Whoa, people spent money based on my advice!?! That makes me nervous...
ADodson obviously has wicked down pretty well, but hey, I feel like writing a long ass post of somewhat obvious observations, so here ya go! Sorry if some of this is too obvious.
With the right exposure -- as he said, a faster shutter speed made possible by a much lower (in number) aperture -- those photos would be useable, I think.
Don't be afraid to crop the shirt out of them. If you go too far, the resolution will be too crappy to print. Our paper prints photos at 200 dpi, so when I'm working photos in photoshop, I check "Image Size" after a crop, make sure the the DPI is set to 200 and make sure the size of the photo is still appropriate. A D100 doesn't pack enough megapixals to give you a ton of cropping leeway, but you definitely should have some. Shoot at the highest quality you can to make sure you maximize those opportunities. I shoot at large fine Jpeg. I think a lot of pros shoot RAW, but those are huge files that eat up card space and require an extra step in the editing process that often stumps my computer, so unless I know I have the time, I don't do it.
An example of how I'd have cropped your photos, the third one down, with the basketball player throwing a pass to a teammate, I'd have cut about an inch off the right side, so the guy throwing the pass and the defender are pretty close to the right edge of the photo. I'd have cut across the top just a little above their heads, and down the left side somewhat along the teammate's back. That will eliminate the massive distraction that is the stands in the background. Keeping the teammate in the photo, even if he's way closer to you and out of focus, also allows the photo to tell a little bit of a story -- the guy's throwing a pass. The other guy's about to catch it.
Here's an example of one I took that I was proud of that reminds me of your pic. It was getting dark, so I was probably geared all the way up to 1600 ISO on my D70s and it's a little grainy. I was able to get out on the field because it was a practice. I wish it were brighter, and that they guys weren't wearing white to match the snow, but, I think it's an OK example of a lucky shot combined with good framing and cropping.
I suggested that 50mm because it's relatively cheap and the aperture goes so low, down to f1.8, which allows you to take workable shots indoors without a flash. It is probably shorter than ideal, however, so you have to be in the right spot to use it best. It looks like you're in a good spot on the bottom photo, but don't use a shot when she's out where she is. Wait until she drives down to the baseline like it looks like she's about to. She will be much closer and will fill your frame much better (and she'll fill your photo when you crop it a little bit) and the defender will be trailing about a half-step behind her. It will make a much better photo than a side-view shot of her dribbling in front of an opponent. Side-view shots, like that bottom one and the one with the kid about to dunk, are often hard to crop well and often not very good. Get in front of the action, so the players are coming at you and you can see faces. That's all about getting in the right spot, though, and picking the right photos. Get under the basket and you can usually get good shots of people driving that baseline or charging down court with a steal. You also know there's always going to be action under the rim, so you can get good, tight shots of people grabbing a rebound with defenders hanging on them, or going up for a layup, or someone trapped by the defense down low. Think about where the most action is, where you're most likely to get a good, close shot of someone, and camp out there until you get it. Once you have something good in the bag, you can try to get a little more creative and go elsewhere in the gym.
Shooting soccer or football or something with a 50mm is very tough, but it's not impossible. I'd stand along the sideline and just wait for the action to come to me. Eventually someone will come charging down the sideline with a defender in tow, and run right past you. A longer lens will give you many, many more opportunities, but if you're really trying to make it happen with a short lens, you have to be ready for the chances you do get. A safe shot from something like with a lens like that that would be a coach yelling at a player, or a sideline reaction to a big play, something you can get up close to. Sometimes you can stand behind the endzone at a football game and hope for a good shot of a running back breaking through the line, or a good post-score reaction. That wouldn't work so well in soccer because they're most likely going to score from further away. Again, with a short lens, you're going to have to be in the right place at the right time, so think ahead.
Also, don't hesitate to take a lot of photos. A LOT of photos. There's a setting on the top, left side of a D100, under that dial. "S" is for single shot. The squares stacked on top of one another if for fast shooting. Set it to that and when that girl in the bottom photo starts toward the baseline, keep the focus square on her and fire away. I took 850 photos yesterday at an event, which is flat out embarrassing, just absurd and a pain in the ass to process, but my way of making damn sure I end up with some options I like.
Also, you mentioned hoping to get a 70-300mm lens with a light meter. That D100 already has a light meter. When you look through the viewfinder, it's at the very bottom. It looks like this: + 0 - with little dots between and hashes leading one way or the other underneath. The "0" is what the camera thinks is perfect light. The - is too dark and the + is too bright. To do this job right, you need to be shooting in manual mode. So set your ISO to 200-400 if you're outdoors in the daylight, or 1600 if it's dark or you're indoors. Then adjust your aperture (f-stop) and shutter speed. Keep the aperture as low as possible (f1.8-2.8, as ADodgen said, with that 50mm) Try not to go below 1/200 or 1/250 on the shutter speed if at all possible. Adjust those until that hash mark gets close to the "0". It's probably OK to be a hash or two one way or the other, but try it and make sure it's working, and adjust accordingly.
Anyway, maybe that's a lot of obvious advice, or plain ol' bad advice. Hope it helps some, though.
