The Solution to the Problem has finally been installed in production. It took a while since I wanted to benchmark different configurations well before doing the actual installations. Enough space for backups for some time! I'm now able to store database dumps and transaction logs for much longer than a week, and there's also plenty of room for database growth.
The new disks are faster then the old ones, and they're a bit cooler, too:
4 1TB SATA disks set up as a RAID10 array, giving 1862.62 true gigabytes of space.
I've also ordered more of the same disks for the live aprs.fi servers.