There are three case with some isolation, that I am aware of:
one tested right here in pages: http://tinyurl.com/3so3
is the Antec Sonata which is promised early in January.
Another Antec case, similar to the Sonata, with (much?) more restrictive fan grills but you can Dremel your way to happiness. Picture is here: http://tinyurl.com/2ii3
Info is here: http://tinyurl.com/3so5
But then again, you said that you didn't want to have to do mods...
The third is an Evercase, similar to to the Stealth computer, also reviewed in these pages. You can see a picture here: http://tinyurl.com/3so7
and info here: http://tinyurl.com/3so9
I'm pretty sure that the larger, squarish holes in the HD cages have rubber grommits that go into them.
Now, I'm sure that there are other cases, but these ones are quite reasonable, especially the Evercase...add a Seasonic or Nexus PS, and you are off to silent running. The Antec's come with very decent power supplies.