To really shine you'll need a full frame sensor. Granted, I'm from the Nikon camp, but I went from a D300 (maybe a little better than your t2i) to a D700. With my 70-200 f2.8 lens I've taken thousands of indoor volleyball pictures. I've settled out on F4.0-4.5 if I can get away with it (to give some depth of field - i.e. forgiveness on focus). So, an F4.0 lens should suffice in most situations (my 2.8 also doubles as a portrait lens). In very badly lit gyms I'll open it up more. I typically end up at ISO 12,800 but sometimes go to 25,600 (max for D700) to get at least 1/500 shutter speed. With some Noise Reduction in Lightroom, the D700 gives 'good-enough' pictures even at the highest ISO. We're not talking Sports Illustrated quality, but over 30 of my pictures landed in last year's yearbook (I donate them freely). Parents also love me since I post them online (for free) and only one other parent that I know of has the equipment to match mine. I also use the same camera for outdoor, night-time soccer games (another typically terribly lit venue).
I realize you have Canon equipment, but I hope the above helps in general. Canon certainly has equipment that matches Nikon's.
If you're interested in the quality of the pictures using the above setup, feel free to check
www.beenphotod.com. You can navigate to sports - etc... from there.
