forgive me for not rememberin the name at the moment, but there's a PCI card theat like 200 bucks that is basically a RAID setup....works with two ATA drives...not sure if you can set it to morror rather than stripe, but if you go to a local apple dealer they can probaby tell you who makes it, or if'n you email me I'll try and dig it out..
excepting that, you can always do what i useta do, just bought a second, larger drive and once a week just option drag the one drive onto the other...slower, but you can do it and go away...
:ryan