How to store small amounts of data in the most durable way

I still think a very reliable off-the-shelf NAS would be the best solution. Although NAS's are usually Linux/BSD based, they provide a friendly web interface and hide much of the unnecessary detail.

If you do opt to build, a full Linux install may require more maintenance than a professional photographer would want to worry about. I would recommend either Windows Home Server or FreeNAS.

Superuser has a couple recommendation threads for both options: Any recommendations on a NAS for a home-super-user? https://superuser.com/questions/4893/homemade-diy-nas-solution

Finally, my own recommendation: I've used a D-Link DNS-323 with two 1TB drives for 1 year and have not had a problem.

The simple answer is: multiple copies. Whatever else you do, don't trust any single media, location or service.

Personally, I currently use external (USB-connected) hard disks for backup purposes. A 2-3 TB drive can be sourced quite cheaply and will provide you with plenty of storage capacity not just for the time being but also account for any reasonable future needs. At the physical size of roughly a large paperback book, it will easily hold the content of 200-300 movie DVDs without further compression. Buy two, or three if you are paranoid, preferably one of which from a completely different manufacturer (might want to mix Seagate and Western Digital, for example, since they are unlikely to use disks with the exact same design or manufacturing defects), and keep at least one of them in a physically separate location - a bank safety deposit box is a relatively cheap alternative that will give you physical security as well, but even just keeping one copy at work or at a friend's home will almost always work just as well. If you can arrange to be able to refresh an off-site copy without bringing it to your own location, that is even better. If any of the content is privacy-sensitive, keep that in mind when planning how to handle off-site copies.

Also keep in mind that the amount of data you are talking about (300 GB counting as more or less "irrecoverable", another 500 GB "nice to keep" but which in a pinch you could probably get from other sources such as second-hand movie DVDs) is not really all that much. I currently have a grand total of about 100 GB of digital photos alone, and it's not hard for me to add during a single day some 10-15 GB to that - and I have done that on a few occasions going to events where I have had reason to take lots of photos. Many of those photos are of in various ways questionable quality, many are mundane (nice to have, but in a pinch there's nothing truly special about them), but some of them actually are irreplacable from a content point of view as well as actually of good quality. For backup purposes, though, I treat them all the same way: multiple copies. I've had a few hard drives fail on me and while a few times I've lost data I would really have liked to keep, overall this strategy has meant that I can restore the most recent backup to a new drive and be on my merry way. If the live copy fails restore the backup to a replacement primary drive; if the backup drive fails, get a replacement backup drive and make a new backup.

If you do go the multiple storage media route, too, remember to keep checking each for signs of degredation. It's fairly quick and easy to do a SHA1 hash run over all the files on a drive and compare the results, as well as storing the list of hashes itself in multiple locations. That way, even if you get read errors at some point, you can determine which copy is "good".