Just use Partimage - its an open source program designed to do exactly this and it has network support.
If you're using it to backup non-root partitions then you can just run it from within your Linux distro (after unmounting the partitions first). If you need to backup the whole thing including the root partition then you'll need to download one of these LiveCDs which has Partimage on it and boot your machine off it:
(smaller contains only essential system rescue tools)
(larger - aims to be complete Linux system on a CD)
Also keep in mind that backing up from one hard disk to another in the same machine is not a safe or reliable backup method at all, since most data loss scenarios are just as likely to delete stuff off both hard drives (eg. rm -rf /*).
OTOH if you're doing it for system redudancy (ie. if hda fails you can just boot the machine up with hdb) then that's exactly what RAID was designed for. Its simple to setup Software RAID1 (a simple drive mirroring setup) in Linux and is far more likely to work properly than trying to copy the drive image every so often.