In Leopard it appears you have to click on the Advanced button first to get to the TCP/IP tab, as illustrated on this University of North Carolina help page.
I also noticed on G5 Center's Internet page this tidbit:
Pro-Tip: Tobias, key developer of Leopard-Webkit, recommends disabling IPv6 in the Network system preferences for your internet connection/port on your Mac if you experience hangs when loading webpages. I can confirm that this leads to a night and day performance change for my G5. WebKit is smooth as silk now.
So disabling system-wide sounds like a good option to experiment with. Just be aware, things like Bonjour or other system services could break, but re-enabling IPv6 is quick and easy.
Que the network IT people saying, "Don't disable IPv6. Fix your @#%^&$ing network!"