Dance Photography - camera for fast action and low light

Hi, I've been taking dance photos for a while and overall I'm happy with the images I capture but I would like the photos to be sharp and crisp. I struggle with fast action shots (ballroom dancers) and generally not optimal lighting.

I've been shooting with an Olympus E-510 and Zuiko 50-200mm f2.8 - f3.5. I think this is a good camera but I may be pushing beyond what it is designed to do. I'm covering the full dance floor and needing to change the focal point very rapidly, there really is no time to change setting.

Does anyone take action photos (not necessary dance) where you're dealing with similar combination of action and lighting ? I would be interested in hearing what camera/lens combinations you are using.

Thanks, Marian