I'm always looking out for hi-res textures as this is the simplest way to make Orbiter look better. I haven't seen any for the default ISS though.
You can have a very realistic and really nice looking ISS with these add-ons:
[ame="http://www.orbithangar.com/searchid.php?ID=5633"]ISS AtoZ[/ame]
and
[ame="http://www.orbithangar.com/searchid.php?ID=3737"]International Space Station v.3.2[/ame]
Viewing the station the frame rate drops to about 10 fps on my old PC so I don't use it. But it does look amazing.