Presuming that you have your own Internet Router...
... once you can get each Mac Onto the Internet without re-cabling, you cabling for File Sharing is complete.
Enable File Sharing on the G5. N.B. "Sharing" in the Mac OS X sense means "Making available for everyone". Once available, you just use it.
on the other Mac use Finder > Go > Connect to Server. Enter a username and password that is Valid on the G5. A pseudo-drive will mount (like an external drive).
On older versions of Mac OS X this gives you access to whatever that G5 User has access to. On newer Mac OS X, you specify what access is allowed right in the sharing panel.
Wireless is a little slow for "near Hard Drive" experience with File Sharing. Gigabit Ethernet with jumbo frames does not seem slow at all, but may require a Gigabit Ethernet Switch in some cases.