you've answered the question youself...
"an Athlon 1600+ is basically a type not a speed and its actual speed is probably nearer the PC's own idea"
the athlon 1600 is not meant to run at 1600Mhz, it's meant to run around the speed that your pc states. fsb to 100 from 133? i'm not sure but i think the xp1600 athlon is a 100fsb setup???
someone who knows more about athlons will correct me and tell you better.