I've been thinking about this sort of question for awhile ... and have made some hardware investments.
I have a couple of old 2GB SCSI drives, if you're interested
Okay, what I did for my G5 desktop was to find an external enclosure that had USB / FW400 and which installed its hard drive (PATA) on a removable tray. I bought 1 enclosure + 3 trays so as to give me the proper triple-redundency in data backup. My original cost to set this up was around $150 for the enclosure + trays, plus three 3.5" PATA HD's at $100 each. At the time, to spend $100 for a bare drive resulted in around a 250GB unit; today that would buy 500GB drives instead.
While its been good about it is that I have a rock-solid system for conducting periodic backups. Of course, what's not been so great about it is that I still have to remember to do them. Also, because it uses PATA and not SATA drives, as I upgrade my internal HD's, I'm not able to recycle them into this configuration. As such, if I were to do this same exact approach today, I'd look for external removables that used SATA. I'd also deliberate using an interface that's faster than FW400, which brings me to part II.
Part II is where I'm looking at heading in the next couple of months.
I've thought about eSATA, but eSATA is not a ratified standard, and in comparision to FW800, it offers no real speed advantage unless your storage is in a RAID 0 or 10 configuration. As such, I'm looking at adding my next major increment of storage to my home network through a Network Attached Storage (NAS) drive, and the one that's on my short list is the 1TB by LaCie for just over $300. This would be hooked up primarily via Gigabit Ethernet, which I'll be able to reach wirelessly for my laptop and wired for my desktop.
What's nice about the LaCie is that it is true Gigabit, not just 10/100, and it also has a USB port which I believe that I can daisy-chain another HD off of, which means that I'll be able to recycle my existing external w/removable trays hardware so as to make a second (& third) backup for redundency.
BTW, I believe that the Apple Airport also has a USB port with which you might be able to hang off a USB drive which would afford network access. Granted, USB isn't fast, but a lot of this comes down to the question of "what are you using this additional storage space for?" My opinion is that priority #1 should be for backup, whereas #2 would be for keeping 'more stuff'. Overall, I'd recommend that you include your long term backup plans while making your decision as to what precisely to get.
-hh