The problem with using external drives with mixed platforms is that you are usually stuck with FAT32, with it's file and partition size limitations. I just bought one of the new Linksys NSLU2 NAS storage controllers. This cost $99 and I added a 250Gb Maxtor drive and a 3.5" enclosure for a total cost of $310. It only took a few minutes to set it up on my home network, and is accessible from all my Linux and WinXP systems as well as my Powerbook. It's not a quick performer given it's USB2 drive interface, but it's great for backup or storing files that you might want to use on any system (MP3s, images, etc). It also has a backup utility built in that can be configured via it's web interface. It supports up to 2 drives and formats them with an EXT3 filesystem, but because the Linksys is effectively the fileserver, it's accessible from all platforms.
The lack of file and partition size limitations means you can store huge archive or movie files with no problems. It's a great solution for me anyway.