Hi Derek, I'm glad you like your 17-85mm - it really is a great all-round lens for Canon EF-S bodies like the 400D.
It seems there's quite a few people asking how the 24-105mm would perform on a cropped body like the 400D, so I'll look into doing some tests in the near future... watch this space!
I have already tested the 17-40 L on a cropped body though and you can find the results in our 400D lens upgrade group test article here:
http://www.cameralabs.com/features/Cano ... s_upgrade/
I can't speak for the Tamron 17-50mm though as I haven't tested it.