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with every camera, the lower the ISO the better. I can tell the difference between iso 200 and 100 on 7D at 100% before noise reduction. that much said, it is very usuable to 1600, and even at 3200, but have to be more careful with selective noise reduction.
there was a very good article from a birder, comparing the detail produced by the 7D and 1d3, and found even though 7d has slightly more noise per pixel at a given iso, it produces more detail. some say 7D has the second best autofocus in canon lineup (behind 1d4). many say a properly funcitoning 1d3 or even a 1d2 i better.