The ISO you need really depends on the lighting in the venue, the aperture of your lens and how fast you need your shutter speed to be.

One place I worked recently required ISO 3200 with an 85mm and using aperture settings from 1.4-2.0, while the place I'll shoot this weekend will require ISO 6400 and I still won't get a shutter of 1/500.

A crop body refers to the sensor size. Full frame (FX) like the D3 and D700 or Cropped (DX) like the D300 and most others. A smaller sensor gives you the appearance (Illusion really as it amounts to little more than a digital zoom - but that is way over simplified of course) of more reach because it will give an effective focal length multiplied by the crop factor.