I know that this is probably going to sound like a smart A$$ answer, but it is not intended that way. I use the lowest ISO possible that allows me to maintain the necessary shutter speed to get the shot. I can post process for noise, I can't post process for blur caused by too slow of a shutter speed.
Indoor sports photography is one of the biggest challenges in photography IMO. It is purely a compromise of lens speed, shutter speed and ISO to get a proper exposure. You have to decide where you are willing to compromise.
One of the most important things in shooting sports it to know the sport you are shooting. Not only the rules and how the game is played but who the participants are and what their strengths and weaknesses are. If you do that you can anticipate the action that will come and have the best setup to get the shot.
I've never shot a cage fight, but 1/250 of a shutter speed would seem to be the bare minimum to freeze the action. For most of my glass I would prefer an f-stop of 2.2 as I tend to use primes for indoor sports over zooms. This lets me keep my ISO down and still allow me to stop down my lens a touch for more DOF and sharpness.
For the basketball shots at the collage I shoot for I am getting away with an f-stop of 2.2 with a shutter speed of 1/320 at a 1250 ISO on my 40D and 30D. Other places I have to bump my ISO up to 1600 to get the same f-stop and shutter settings. Those are my minimums for basketball, as I am pleased with the results I am able to achieve.