First, Mac OS X 10.2 supports PPTP tunnels, not IPSec. I don't know if that makes much difference in what you're trying to do or not - just be aware of it.

Second, I've never thought of using an IPSec (or PPTP) tunnel to encrypt ALL traffic. I've only ever used it to encrypt specific subnets - but I guess it's possible.

What you'll need is a PPTP server running on a wired Mac on your network. PopTop is an open-source PPTP server that may well work (it lists Linux, Solarix, FreeBSD and OpenBSD, so you might be in luck)