The 1D MK III has many improved features over the 1D MK II. I used mine at ISO 3200, there is noise, but its manageable.
ISO is better, but don't expect miracles.
A larger LCD, supported on modern computers, live view, its a great camera.
However, for a big jump in high ISO, go to a 1D MK IV if you can swing the extra cost. ISO 6400 is pretty reasonable on the MK IV, and I've used mine at 12800.
A 5D MK II or 6D will beat them out at high ISO though.