ISO 640?? Did you mean ISO 6400?
6400 is pretty good too but for it to hold up you need to get the shot nearly full frame. For most wildlife especially birds you often have to crop which magnifies the noise. I shoot the 5diii up to 1250 on a regular basis and the noise is minimal even with cropping. After that I find crops really take an iq hit.
People think high ISO is just for low light shooting. On the contrary high ISO lets you use real fast shutter speeds and higher f stops in daylight. Done right, you can over expose slightly and pull the exposure back down for even less noise. The only thing better is the 1dx and I found it to not be that much better in the iq area.