I have a single Mac Mini running as a makeshift NAS (Hate all you want about Apple products, but the 11 watts on idle is pretty amazing). The Mac Mini has a 64GB Crucial M4 and a 500GB internal hard drive, which is really enough for my needs. Software-wise, it's running as a Plex and file server, as well as running software for my home security system. I upgraded the RAM to 16GB, which is currently just being used as a cache for the hard drive.
For any large data archiving, I use an external 1TB drive that I mostly just use for older data. I barely plug it in, since I've had an external drive die on me before, and I don't want to "use up it's lifetime." (Paranoid, I know...)
All my remaining desktops and laptops only have SSD's installed. My main gaming desktop is using a 128GB Samsung 840 Pro, and currently have less than a gig left (games take up huge amounts of space ).
My laptop is using a 128GB Crucial M4, which is also running out of room.