here is what I've planned for my backup server:
2 x 80GB (1 of them for redundancy)
2 x hard drive fans

The rest will be old hardware and a hard drive for the OS (FreeBSD).

Key items:
I'll likely have remote servers rsync data (or just run them off its crontab).
Monitoring hard drives with smartmontools and shutdown if its check fails.

Does anyone have any comments/suggestions? I haven't setup a backup server before so I'm not sure how it will turn out (also not sure if rsync is ideal)