I agree with Matthew. We also use two external hard disks alternately.
For making the backups, we use LaCie's free backup program Silverkeeper. Efficient, fast, lets you schedule backups and keep different pre-configured backup sets. Has been working flawlessly for more than two years here, and is compatible with the latest Panther release (10.3.2). We see no need to pay for Retrospect.
Go to
http://www.lacie.com/silverkeeper/
Alas, it's not compatible with tape drives.
Regards,
Thomas
AC4.1-AC26SWE; MacOS13.5.1; MP5,1+MBP16,1