Then speaking generically, hard drives, when not serving in a capacity where you aren't depending on them for daily operation are quite reliable. One, fairly inexpensive option would be to use a separate hard drive, such as one that connects via USB to a PC on the LAN. Then you can use a daily incremental backup utility to make an image of the system(s) that you wish to back up. The prototypical backup method is tape, but that will run at least 3x-4x the cost of the hard drive approach. You could pick up a USB 1.5 TB drive for about $150 (US).
The most basic of the incremental backup utilities that I am aware of is rsync and there are a lot of front ends for it. You didn't specify what your OS requirements are and this will impact your options.
As you are looking for about 500 gig of storage capacity, this pretty much rules out optical media such as DVDs. If you do consider them, be sure to get archival quality ones. The best seem to be Taiyo yuden (spelling) DVD+R if they are the ones made in Japan (only).
I also suggest that you take a gander through the last several pages of posts in this forum as this question has come up in the past and you could gain some insight from those discussions.