If only Opeth made more prog-rock songs like this, I would instantly be a huge fan.
Have you even heard all of Opeth's Damnation album? That entire album is prog-rock. If you dislike Opeth just cause their death metal vocals, you are missing out big time. Check out these songs if you haven't though, they're all clean:
Face of Melinda
Still Day Beneath the Sun
Patterns in the Ivy
Patterns in the Ivy II
Hours of Wealth
You should really, really consider checking out all their stuff though, I would never listen to any of the heavier stuff of theirs at all, until I finally forced myself to, and now today they are my second favorite band. :3