There are three different ways to get at that, one of which you're already trying; you can use a VNC client (basically Chicken of the VNC is the only decent one right now) with the built-in VNC server in ARD; you can use the same with a 3rd party VNC server (I like Share My Desktop), which gives you somewhat more control over settings; or you can buy a copy of ARD and use that.
I'm assuming you're currently using CotVNC with ARD; this has worked fine for me in practice, but has the major annoyance that I can't seem to get a proper connection at 256 colors without changing the bit depth of the host computer (which looks wretched). It also doesn't offer all the "speed enhancing" things that a full VNC server does.
ARD is way expensive, but I assume it works, but I'd probably just go with a VNC server since that's free.
Regardless, the things that can stop this from working are:
1) The IP address your G5 is getting isn't accessible from outside the local network. If the internal network is "isolated" from the outside world (say, IP addresses in the 192.168.x.x range), then the only way to get to it will be if you can get a VPN connection going. This would cause your laptop to appear as if it was on the local network, and let you connect via the IP address of the G5.
2) The firewall could be blocking the appropriate ports; either the G5's firewall (I don't think it automatically opens the ports for VNC even if you use the ARD server), or if there's a firewall on the whole network. That being the case, you'd need to open the relevant ports on the firewall to make it happen.
3) Make sure you're using the correct password and username to log into the G5--if you're using the ARD VNC server, it has its own password and login for VNC, NOT the username you're trying to log in as--you'll need that as well if it's not already logged in.
4) If the situation is #1 and you don't have an externally accessible IP address or VPN access, if it's a small network you may still be able to get one of the ports forwarded to your G5, which you can then configure on the G5 to be the port the VNC server is listening to. This will probably only be an option if it's a small network with a basic router.
Hopefully something in there points you in the right direction.