Your post is a little vague, you state that you have been doing sports shoots for a while and you also state that you are new to digital photography. So have you been doing your sports shoots using a film camera, and was the camera a SLR? Also just what do you have in mind when you say you want to branch out?
If branching out means shooting from the sidelines and capturing live-action sports shoots then inexpensive lenses aren’t in your future. The type of sports also has a big influence on the lens you will need. For indoor sports like basketball, the lighting is seldom conducive for photography so you need a very fast lens, and fast lenses cost money. A popular lens is the 70-200mm f/2.8 and that lens sells new for right around $2400, and not a whole heck of a lot cheaper used. For outdoor sports like football, baseball, and soccer, during the day light may not be a problem but distance can be a problem and a telephoto lens is almost essential. Just look at the lenses the professionals are using next time you watch a game, all those great big white lenses are Canon lenses, Nikon lenses are harder to spot because they are black, and some of those are approaching $10,000. Many amateurs find that the 70-300mm VR lens works for them for daytime outdoor sports and that lens costs right around $580. The 70-300mm VR might work for night games at a stadium that has really good banks of lights, but being f/5.6 at 300mm it isn’t very fast and not well suited for poor lighting conditions. Flash is generally not very effective at the distances you will be shooting and generally isn’t allowed at indoor sports.
The D90 has some very nice features for sports, such as a faster frames per second (fps) rate, which can be very beneficial shooting sports. The dedicated controls mean that you can change camera settings quickly when you need to. There is only about a $130 difference between a new D90 and a new D5000 and the ability to use both AF-s and AF lenses on the D90 might prove to be a big factor for someone looking to buy used lenses. There are some great used AF only lenses that will auto focus on the D90 but not on the D5000. The 18-55mm “kit” lens on the D5000 is not going to be all that useful as a sports lens; the 18-105mm “kit” lens probably won’t be either but in day-to-day shooting the 18-105 gets the nod. I would also say that if you are really serious about learning digital photography the D90 has more potential as your skill-level grows.
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While amateurs change the camera’s settings; many Pro’s prefer to change the light.
Brooks
http://bmiddleton.smugmug.com/ 
