OK, those programs didn't actually make US games work on my xbox without the chip - not entirely sure what they do....
But I did manage it anyway, using this:
http://www.xbox-scene.com/articles/configmagic.php
and just not quite following the instructions - changed my xbox region to the US one, obviously, and it's working now without problems.
Thanks anyway for the help, and again, hopefully this will hope anyone else looking for the same thing.