I'd suggest that partitioning the drive would be counter to your purpose. FAT32 is fine for both operating systems to use to store disk images.
While I use Ghost on my PC, something as simple as Linux's dd command should work on both platforms; you just need appropriate boot CDs. I'm not aware of any disk imaging software for MacOS, although one would hope it exists.
You should carefully consider archiving your back-ups to a more stable medium, like DVD, every once in a while. Although the possibility of a double hardware failure isn't too high, the probability of user error is great.