I have 180 GB in my desktop, and 80 and 100 in my laptops respectively (well my mother's, but I use them mostly). My external is 250 GB.
The desktop doesn't have much on it, about 10 GB worth of replaceable stuff, mainly torrents. The larger laptop has a few HD movies, which I delete once I watch them. The small one is empty, just a few school docs. My External has the complete BSG, season 4-5 of Atlantis, and the SG1 movies. It has a few distros from time to time, and usually some school work.
I only really have about 30MB of irreplaceable stuff, as well as my 250MB Funny Pictures folder, now 1800 strong.