I don't do computer boot images, if I need a emergency boot disk set aside then a separate external HDD would be just fine.
Let's see here:
- Archives (usually OS 9+, like 10.5.8 Combo, 10.2.8 Combo, older iTunes downloads, old Handbrake versions, the lot) ~14GB
- Book library (mostly PDFs, and kludgy image dumps rather than OCRd stuff) ~11GB
- Documents ~2.5GB
- Games.dmg (mostly older stuff my 'mini won't even run) ~1.3GB
- iTunes backup (Music+audio book=~1.7GB, rest is iOS backups, as I backup old versions of .ipas, it has been useful a couple of times) ~21GB
- Movies (various) ~91GB
- Classic Mac archives (system folder images, this and that) ~4GB
- Pictures ~2GB
- Slipstreamed XPSP2 .iso backups* and Windows archives of everything useful I find for Windows** 4GB
Free space on 250GB FAT32 volume: 77.53GB. If I put copies of OS X install DVDs, which would be nice since I have no confidence in Apple's future plans to maintain machine-specific 10.4.6 install disks for this machine, I'd best do it myself. Or 10.5 disks, but those should be around for awhile.
I'm thinking a stack of 15 MO disks should cover me just fine. Some of that stuff should ~really~ be backed up solid, that's what I figure the MO discs would be used for: the classic Mac stuff, archives, images, documents, et cetera.
*: This was before I learned that SP1 will not work properly in a 'mini
**: Everything I find useful for Windows gets backed up. Fortunately that isn't much, but as I have to spend lots of time finding stuff for Windows that works, I find it best to backup that which I find works so I don't have to hunt all over again for it.