Hi,
I would suggest that you go for the 7D over the 50D if you want to shoot sports because of the increase in AF cross points and improved tracking abilities. The most common lenses for indoor and outdoor sports (without going into the big lenses) are 16-35 2.8, 24-70 2.8, and 70-200 2.8L IS. Optional 1.4x or 2x for extra reach outdoors depending on situation. Unless you have a lot of available light or the more capable Pro level bodies, extenders indoors are pretty much useless because it will stop down your apertures too far for non-flash indoor use and will sacrifice on some image quality.
Commonly known as the "Holy Trinity", these zooms will also cover all your general purpose shooting needs for the most part. Tripods are not allowed in most school gyms and football fields unless you have permission from the coaches to be there.
Flash is often times not allowed indoors. Outdoors as well if you're on the sidelines, but flash is absolutely useless when you're in the stands; it won't reach. Monopods are allowed but they're only good if you plan on using a 70-200 2.8 or up. Just loosen the tripod collar so that you can easily rotate your camera from horizontal to vertical shots, and back. Just remember to tighten the collar again when you have to move or place your setup down.
Fast primes are just too cumbersome to be switching out all game long. And depending on where you are, will not have the adequate reach. It's easier to switch out two lenses constantly than a whole mess of them. You can miss a lot of shots by switching lenses in and out if your not proficient at it. One of the advantages of shooting with a crop sensor is that you can shoot wide open, and not worry about too much vignetting (non crop style lenses on dx bodies). But shooting completely wide open on most lenses can result in soft images (read and review your lens' manual), chromatic aberrations, vignetting, and difficult metering. It's best to stop the fstop down a step or two for best results, unless you prefer spending a lot of time Ps in post.
The 17-55 2.8 IS is a great general purpose zoom and pairs well with the 70-200 2.8. If you're going to shooting from the stands, recommend you save up for the IS version. Those stands can vibrate and move when excited fans are jumping around, or from you shivering in the cold at night.
Definitely learn how to do a "custom white balance" when working in artificial mercury and sodium vapor lights. It will play havoc on your color tones that Ps will not always be able to balance out by clicking on the magic eyedropper.
Oh and yes, if you figured it out. I'm a freelance sports shooter.