Will a modern Time Capsule work with 10.5? While at home for the holidays, I noticed that the old iMac is probably on its last legs. I'd like to get a Time Capsule so that the documents are backed up when the time comes. However, the thought crossed my mind that the old iMac with OS X 10.5 might not be compatible with the new Time Capsule.
The new Time Capsules require OS X 10.75 and later:
http://www.apple.com/airport-time-capsule/specs/

Your best option is to upload your documents online (Dropbox, Google Drive, etc.) as your 10.5 backup on an external drive say, may not restore to the current version of OS X.