sport in low light question??

Hi, when taking pictures of horses running in an inside arena with low light and some dust around, everytime I increase the speed of my camera, to capture horses legs better, crispier, the pictures are too dark, should I try to place myself so light hitting the lens?? am I understanding the principle allright?? it's seems if light behind me, or between horse and my camera, dust is showing badly on pictures...but anwhere I place myself, pictures seem always too dark...

I don't have access to other light then few windows, few ceiling lights and my camera flash...Just want to know If I understand the principle of it...

Here some of the best ones I got...thanks

http://i54.tinypic.com/20uy33r.jpg

I see you shot this at 1/125 f5.6 iso3200. What lens are you using? I would very rarely shoot at iso this high. You could open the lens up to F4 but of course this might limit your DOF especially if you are close to the horse. At 1/125 you still seem to have a bit of motion blur in the hooves and tail. This is ok if you want it from a creative point of view. You'd probably need 1/250 to freeze it really nicely. It would be a good idea to post one of the dark photos so we could see what they look like. I would recommend trying to get the shot when the subject is as close to the light source as possible. Without knowing too much, I suspect you might be limited by your hardware combination as much as anything else. BTW the photo above doesn't look too bad to me from an exposure point of view.