You may want to consider Nikon's 135mm f/2 portrait lens, it will work well for sports, low light and it's ideal for portraits. Otherwise, you could consider Nikon's 70-200 VR II f/2.8, which although it isn't as fast offers more versatility and VR. If that's over budget, you may want to take a look at the previous generation model, the 70-200 VR f/2.8, which will work just as well (assuming that you're using a crop camera, you may experience some vignetting and poor performance in the corners will full frame) and can be found for much cheaper.
Take note that the 135mm is an AF lens, so it won't auto focus on the D3000, D5000, D3100 or D5100 (any camera without an AF motor built in).
Gear: 7 Nikon Nikkor AI-S and AF-S lenses, SB-700 flash, Nikon D7000, Nikon FM, variety of accessories
"There are no rules for good photographs, there are only good photographs."
- Ansel Adams