Dance Concert Action with Flash

[johnrogers45]

I have to photograph a dance school concert this week-end. I did it last year but I'm hoping for better photos this year. Last year I used a setting of 1/250 Av, auto ISO and the 430ex flash on auto. I use a Canon lens 28-135 f3.5-5.6 IS USM on a Canon D50. I find I use the whole 28-135mm range so it is a good fit. I used the auto ISO setting because I could not predict the lighting conditions at the time I took the shot. Also because it is the action I want to capture I use the high speed continuous shooting mode and center focus.
Last year these settings gave reasonable shots but most were f3.5 to f5 and the auto setting giving an ISO of 400 on most photos with a few at 1000 and one at 1600. I am thinking if I fix the ISO and increase the ISO to 800 I may get f7 or better. The shots I took were in focus but slightly soft. The action was captured reasonably.
Can anybody give me an idea if I should use this iso setting (800) or keep the auto setting? Any other tips are most welcome. I have attached a couple of photos for reference.

[hus]

Hi there Johnrogers45 I also shoot at very low light conditions and what I use on my Canon 50D is Manual setting, shutter at 1/250 f4.0, ISO is at 1250 and flash compensation at +1 and on my menu I set the High ISO Noise reduction is set to HIGH, I find canons on board noise reduction does a great job on the 50D. The flash I use is a YN 468 and most of the time I have to use direct flash to light up the subject.
Below is a sample of a pic at those settings, hope this gives you an idea to work off.