The WAP54G is not a router, it is an access point.
Disable its ability to assign DHCP to clients. DHCP can be handled on the ethernet side of the access point by your existing DHCP server.
If you wait a short amount of time you can enable WPA for both clients and access points. As of this date, 6/25/2003 Microsoft has provided the support for WPA for its XP clients, Linksys has provided the support for the WPC54G clients and the WRT54G router. More to come.
But if your firewall is up to snuff, I fail to see what the security problem could be at present.
Even if I could "war drive" and access the router, and even if I broke your existing SSID, MAC filters, and WEP encryption scheme, and even if it gave me an IP on your local LAN, your firewall should block any attempt from me to phone home.
I think you are making a security mountain out of a security molehill.
This is an attempt to reassure you that you should be fine with your existing setup.
If you can detail a particular security concern I would be happy to help.
Best.