The Belkin is a 4-port 802.11n-capable wired and wireless router, and I'd expect that box can act as a firewall. If there's another firewall between the Belkin and the Westell, you'll have to sort that out.
If you're connected to the local WiFi but not to the Internet, then the Belkin isn't configured correctly on the WAN side (and given it's throwing address errors, it's distinction possible the LAN side is wrong, too), or the Westell modem isn't configured. With some of the Westell boxes, it's fairly easy to dump the required WAN-side settings with the use of the reset button. Comcast support should know how to (re)establish those settings. Various of the Westell devices can also act as DHCP and firewall.
The Westell F90-610015-06 looks to be a firewall-router-DHCP-NAT box, so that definitely needs to be configured correctly to get onto the Comcast network.
The Belkin can also likely be a firewall, and you probably don't want it running as a firewall NAT box. It should probably be set as a WiFi router, and with a range of DHCP addresses that don't overlap those issued by the Westell, or as an Access Point.
Call in somebody in your local area that can help you configure your gear. Start with Comcast, and ensure that the Westell box is configured correctly. Then start on the WiFi, and that's probably easiest if you either carry the existing WiFi and router gear into a local computer shop, or if you work with the shop to configure your existing or new gear on the bench and then carry it home. Or an on-site visit. I'd expect that most any local computer shop or local home and small-business network consulting service has some experience working with WiFi hanging off the modem for a Comcast connection, too.
And FWIW, this is the Mac OS X Server forum, and I don't see the server-oriented variant of Mac OS X listed anywhere here; this isn't the best forum for Tiger-vintage client WiFi networking. Probably better to continue this discussion over in the [Panther 10.3 and Earlier / Getting Online|http://discussions.apple.com/forum.jspa?forumID=738] forum, though - if you're on the house WiFi network with the AirPort - this looks to be centrally with the Belkin WiFi or the Westell box and not with whatever G3-class Mac is in use here.
I'm guessing that most of what is above is probably unfamiliar territory; WiFi with a mix of gear isn't as easy as it is with an AirPort Extreme or other Apple base, and even getting from the AirPort Extreme or Time Capsule or such over to an arbitrary Internet ISP can sometimes be challenging. Add in a Belkin and a Westell and Comcast and things get more "interesting." Which is why I'd suggest recruiting the assistance of the local shop or of a local consultant.
