I get most of mine online, so a lot of them are single scenes, not full movies. I have a 300GB external hard drive that's full, and if you assume a compressed DVD is about 1.5GB, that's the equivalent of about 200 DVD's.
Funny thing is, I rarely go back and look at many of them. Its more of a compulsion to collect than a desire to watch. I see a star or peculiar fetish that turns me on and I just get obsessed with finding every scene I can. Then eventually I get bored with it, throw what I've found onto the external drive, and move on to something else. Every once in a while I'll get an itch to watch it again and pull it back out, but generally its forgotten. Yet, for some reason, I couldn't bare the thought of deleting it. Weird.