By default this is configured as 192.168.0.1 on 255.255.255.0
In this new house i can run cable, so I've hard wired one wrt54g (ip 192.168.0.5, def 192.168.0.1) in to the netgear and things aren't pretty.
I've switched it over to 192.168.0.*
If I'm understanding your setup right, the problem is here. You've connected together two routers with the same subnet. So if the modem at 192.168.0.1 wants to send a packet to a client connected via the wifi at say 192.168.0.123, where's it going to send it? With no routes configured, and the local LAN configured for 192.168.0.0/24 it will do an ARP broadcast for 192.168.0.123 to try to find it locally and get no response, and think the host is unreachable.
Simplest solution IMO (if possible via admin interfaces):
- keep cable modem at 192.168.0.1/24, connect to wrt54g via LAN
- wrt54g eth IP 192.168.0.5/24, wifi IP 192.168.1.1/24, default route 192.168.0.1, DNS 192.168.0.1
- add static route to cable modem for 192.168.1.0/24 network, with gateway 192.168.0.5
- cable modem DHCP: pick a range on 192.168.0.0/24 net
- wrt54g DHCP: pick a range on 192.168.1.0/24 net