Becareful about curtain shadow at 1/320. In my experience, if a camera is rated for 1/250, you can sometimes get away with 1/320 (especially if it is on the hot shoe directly) but there is some shadowing. Take a picture of a blank wall @1/320 with no ambient light and see if one edge of the photo is slightly darker. Under normal conditions with ambient if you are only using flash to fill this may not be noticeable.
I did indoor roller derby once a week for about a month, trying to get as much shutter speed as possible. 1/320 worked. Slight shadow on the bottom edge.