Alot depends on budget, but a very reasonable starting point for the enthusiast on a moderate budget is a 7d, with the fastest glass you can afford. If MOSTLY doing sports then the extra reach of the 7d (or 1d IV) must be balanced against the better low light performance of the FFs - but I'd stay away from the 5d2 if SPORTs is the priority. AF options doing sports is very tricky (doable, but tricky) and my keeper rates with the 7d & 1d IV are very good. Until we have solid spec's on the 5dIII/X, I cannot speculate there.
If budget is not an option and you can wait, the 1Dx would be the BEST (costliest) of both worlds.
But whatever body you choose, definitely put the most thought into your glass and WHERE you will be shooting.
If mostly indoors (gyms / courts / reasonably close action, typically poor lighting) my first purchase would be the 135/2.
If funds are tight, then (believe it or not) I like the 50/1.4 or the 85/1.8 if you can be close to the courts. (the 85 1.2 is too slow on the AF for fast sports but beautiful if doing more photojournalistic shots and less 'action' shots).
If doing bleacher shooting and funds allow, then the better (BEST) option would be the 70-200 2.8 II.
If you do use the 135 (which I do and love, check out the lens gallery here) and it still doesn't "reach"your target, then you'll NEED the 70-200 2.8 or the 135 with a 1.4 or 2x TC. Again if budget is less an issue then the 200/2 is another thing of beauty, but as a compromise, the 200/2.8 is a bargain at ~800 USD. Be aware though, that shooting primes and sports requires a great deal of practice/knowledge to be in the right place at the right time to get the framing you need, and the 135 and 200 primes do not have IS, so a monopod is very useful.
Outdoor field sports is a whole different ballgame, where lighting is better (but more variable), and many more multipurpose lenses become more practical. 28-300 as a single option, 70-300 or 100-400 as cheaper alternatives.
I'm leaving out the very good 24-70 as an INITIAL lens to recommend now that a new one is on the horizon (at the end of someone's pot of gold), but I do use this quite a lot, and am considering upgrading myself, but until I try out the mark II, I would hold out for now. Though again, if $$ is not the limiting factor, it is an excellent court side choice (more versatile than the 50 prime), The EF-s 17-55 2.8 is another great idea if you decide to go the 7D body as it is a superb all-around crop body lens, and has a niche for sports, if you do use 2 bodies. The 24-105/4 is another nice piece of optics, but for sports, best reserved for the outdoor arenas.
You should begin there with these 2 focal length ranges (< 100 and
135-300 or more), and have 2 distinct perspectives, with enough shutter speed to get all the action.
If keeping 2 bodies is an option, then when you want the group / team photo holding the trophy, the 50 (24-70 or 17-55) on a crop body (with the Nike Zoom
gives you the best chance to get the emotions of the moment without having to change lenses after the winning point and everyone is running around.
Bottom line, think about your camera's location (ambient lighting and your ability to get close physically or optically) before spending your money on the more expensive, faster lenses, but in the end, that is where I would start, good luck and happy shooting.