This thread has lots of good suggestions for storage. I have a distant business relationship with Drobo, and think they're an interesting choice. I have a Windows Home Server as well, and find it to be a step-up from my previous Buffalo Terastation NAS box from a reliability and performance standpoint. I happen to also have a full-size tower and appreciate the simplicity of throwing lots of hard drives at the problem.
However, as formats switched from DVD to Blu-Ray and equivalent HD content, the economics shifted IMO as well. Given the both my Sony TV's, as well as both Tivo's can stream on-demand HD directly (Sony does it without annoying buffering by the way), and considering that I only rarely watch a movie more than once or twice, it's actually become more economically feasible to simply rent HD on demand for $4.99 a shot. There are still going to be a few Blu-Ray discs worth buying to own the content, and more than half of those seem to ship with a free digital copy for import into Window Media Player or iTunes. Even if you own all your HD content on disc now, it's probably worth your while to look into a hybrid model where you rent what you have a passing interest in, and buy/store those few things that either aren't available on demand, or that you have a more long-term interest in retaining.
Oh, and for the porn, a 2TB drive in a large tower should be more than sufficient. Windows 7 and Bit-locker full drive encryption doesn't impede HD playback on a reasonably speedy system. Although it is debatable whether or not 1080p is actually a good thing in some cases. Anyone want to go into business with me creating a unique line of porn-star body make-up to deal with pimples, waxing irritation, and razor-burn?