Try 192.168.100.1 for that modem. If you get to its configuration page, then that's all we need to know to verify that its LAN IP address is 192.168.100.1.
If you are ok with keeping that same IP address scheme (192.168.100.x) for the rest of your network (it's just a preference thing. it doesn't affect how your network works), then you can plug your computer back into your WRT54G router, log back into its configuration page (via 192.168.1.1), and set its LAN IP address to 192.168.100.2 and point its default gateway to 192.168.100.1 (the modem's LAN IP address).
Now you should be able to power off both the router and the modem, wait 30 seconds, power on the modem, wait 30 sec, power on the router, wait 30 sec, and then you should have internet connectivity through your router. Before powering the modem & router back on, be sure you have the modem plugged into the WAN port on your router.
Now as for your wireless settings, if you have already configured your wireless password XXXXX earlier, then you should be good to go there as well. If not, simply log into the router (192.168.1.1), click the Wireless tab, and change the wireless key and click Save Changes.
Another security recommendation is to change the router password (which by default is set to 'admin'). You can change the router password XXXXX clicking the Administration tab within the router's configuration page, then just change the filed that says Router Password (enter it twice to confirm it), and then click Save Changes.
If you have any more problems/questions regarding this issue, please post back here and I will help you in any way I possibly can.