Have your client machine got any DNS settings in them - if not you have two options:
1) Copy in the DNS server addresses that your ISP has given you
2) Run a DNS server on your RH9 box and put that IP in
If you don't know what you're doing then option 1 is easiest - just look at what server addresses are in /etc/resolv.conf on the RH9 box and then copy them into your win2k boxes.
Also, rather than putting the echo... command in rc.local it would be better to edit the line in /etc/sysctl.conf that way if you restart your network interfaces (which seems to happen every once in a while automatically) you don't have to run the echo manually to make it start working again.
Firstly you shouldn't have that gateway on eth1, it isn't needed and actually breaks the rules as the gateway should be within the same subnet as your IP address.
What happens if from a client machine you do: ping www.google.com rather than pinging IP addresses.
If that works then DNS isn't the problem, in which case you need to look for any proxy settings that may be sitting in the client machines that aren't needed or if a setting is in the server that is needed and should go into the clients (I am assuming the server can browse at the moment).
Note down the address that is given at the beginning, and assuming you are getting a result check that this is one of the addresses that is in your windows machines as DNS servers.
If that is all correct then I can only think you have some firewall rules somewhere that are affecting your setup - although by default on RH9 iptables should not affect outbound traffic, only inbound stuff.