will this get around compatibility problems between my adsl modem and the G4?
Not sure about your exact setup, but I run a G5 and Win2K machine on a network via a 4 port ADSL router as jimbo_mac suggested. I can only administer the router settings from the Win2K machine, but other than that it works very well (the G5 recgonised and connected to the network during its initial setup, fantastic stuff :)).
I find each computer occasionally can't find the other one though. The keychain in OS X is a mystery to me as it doesn't seem to help the situation - I get asked for network passwords a lot of the time.
If you want to use your laptop wherever, you might consider an Airport BaseStation and an airport card in your laptop. You can then set up your Airport to act as a router, sharing your Internet access among your machines.
(This is how I set up my cross-platform home LAN. It was pretty easy and very convenient, but not the cheapest way.)