I have much the same setup under FC3 but I don't use DHCP. What I did was to statically configure my router to an IP address, statically configure my Linux machine to an IP address, and then setup the gateway on my Linux machine to point direct to the router.
Can you ping your router? What do you see if you go
in a terminal?
Mine looks like this:
[rylan@StefanLinux ~]$ /sbin/route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
169.254.255.0 * 255.255.255.0 U 0 0 0 eth0
169.254.0.0 * 255.255.0.0 U 0 0 0 eth0
default 169.254.255.100 0.0.0.0 UG 0 0 0 eth0
Note that on your system also there must at least be one UG flagged line - if there is none it means your routing table does not have a gateway defined, and it will be impossible to access the net from that PC.
If you see you don't have a UG line, try typing
/sbin/route add default gw 169.254.255.100 netmask 0.0.0.0 eth0
as root in a terminal.
Of course, substitute my router's LAN address (169.254.255.100) with the LAN address of your router.
After doing the above, try and ping your router and see if you get a response. If you do, you also need to ensure that your system has some DNS servers configured.
In a terminal, do
(at least, this works in FC3)
and see if the file contains anything. It must look something like this:
where the IP addresses of course, are the DNS servers provided by your ISP. If there are no IP addresses configured, simply add some lines beginning with "nameserver" and enter the IP addresses for the DNS servers as shown above.
Once you've done that, also edit the file
(at least on FC3) and make sure it contains
where, of course, you again substitute the IP address on your LAN of your router into the GATEWAY line's IP address.
Hope this helps!