my dual g4 mac started locking up lately; i had some fleeting indications that one of the two disks was not entirely healthy. first i installed julian mayer's smartreporter to monitor the s.m.a.r.t. status and get alerts with email. when it finally started reporting that (sigh) the boot disk was getting close to failure, i brought in mike bombich's carbon copy cloner [essentially a wrapper around asr apple software restore] to make a bootable clone of the failing boot disk. [not wise to wait as long as i have] an hour later, a bootable clone is created and the system is running from the second disk.
phew. both utilities are free (ccc is donation-ware), simple to deploy and are recommended.