When to replace hard disk drives

Telemachas writes: I recently purchased a Dell P4 2.8 GHz swap meet computer with a 200 gig hard disk for a good price and all is working fine. It does not seem prudent, however, to trust my data on a swap meet item. For another @ $ 75.00 each I can purchase new 200 gig HDDs. I would also like to do my first RAID system. I am now wondering how often, if at all, /.ers replace their HDDs? At what time interval(s)? And why (if HDD is still good) ? TIA.