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In order to test VPN connectivity, you should ping an internal SLAC only server; an example would be www-lanmon.slac.stanford.edu
Why don't you support Mac OSX 10.5.8 on a G5.
Regardless of getting VPN to work, note that Apple is about to release Lion (10.7), and will likely drop support for 10.5 (while they have not stated that officially, that has always been the way Apple does things; two versions are supported, otherwise you need to upgrade). And, as you know, the PPC/G5 is not supported in 10.6/10.7, so you really need to plan to replace that system. When Apple releases security fixes for 10.6/10.7 and not for 10.5 (figure August-ish, but that is a WAG), 10.5 will no longer be in compliance to connect to the internal SLAC network.