I have several La Cie external hard drives atttached to both my G-4 and G-5. They are relatively inexpensive. They can be daisy-chained. (And, of course, burning CDs or DVDs. are another way of clearing space on hard drives.) (Elementary, my dear Watson....)

Your options vary from simply going out and buying an external hard drive to getting the physical drive itself and installing it into an enclosure. An Apple Store (assuming there's one near you) is a good place to start, as is Apple online. They usually stock LaCie drives. Office and computer supply stores are another place to look.
Some good advice with buying a new external drive is to check out the manufacturer's warranty. It's good to have the warranty on your side. Then there's the USB 2.0 versus Firewire connection. People argue over how much faster Firewire is over USB 2.0, but faster can be better as it is in my case working with very large image files.
Another external drive option (the thrifty one) available to you, one that I personally use, is purchasing the drive itself and installing it into an enclosure. It involves a little bit of assembly, but gets you more bang for your buck. I usually purchase a Seagate 250GB external drive (good price and an even better 5 year warranty!) online and then pick-up an enclosure from my local computer supply store. After a tightening a few screws and connecting some cables, I have myself a new external drive. This saves me about $100 versus buying the whole external drive ready to go.
To save some extra money look around online and check out what rebates are available on these products. And compare shipping costs. You'd be surprised!
It's whatever works for you. Good luck!