it depends how serious you are about it. it can be as simple/cheap as getting another SerialATA hard drive and software raiding them together using disk utility, or as expensive/high performance as $12,999 to get a 5.6TB xserve raid, and $500 for a fibre card to attach it to your G5. And of course there are many variations in between using firewire 800 or a SerialATA PCI card.
Really though, unless you're super rich and need to burn money or have a specific need for it (like you're doing digital video), your dual G5 is wicked fast and you don't need to do anything to it.