I have to disagree with JohnG.
I regularily use an F2.8 lens for indoor venues with much less light than a gym.
Many pro's I work with use nothing faster than an F2.8 lens indoors.
Also, an F1.8 lens wide open provides so little depth of field that you may find yourself missing quite a few shots due to depth of field / focus problems.
Most of my indoor shooting occurs between F2.8 and F4.
Mind you, I'm shooting ISO3200 most of the time whereas a rebel xt goes to ISO1600.
Something like a Sigma 70-200 F2.8 would do the job, and I've seen many sports photos, indoor and outdoor, from that lens posted to this forum and PBASE.
Of course, there is nothing wrong with getting a Canon 85mm F1.8 lens, as it's a great lens all round.
Also, there is no such thing as an all round "sports lens".
Football could be best served by a zoom with 300 or even 400mm at the long end.
Whereas for basketball, it would be hard to justify needing a lens at even 200mm at the long end.
I shoot inddor track with a 28-75 and sometimes a 17-35 and other times a 70-200 F4, mind you I'm trackside