First off, let's start with apache.
Just to double check, when you put in your local url (
http://192.168.x.x ) you get the default apache page?
If this is the case, then the problem is probable a firewall problem. Pretty much by default mandrake preloads a firewall.
The config files are in /etc/shorewall. For your purpose, you already have a hardware firewall, and to start that is fine. Later you will definately want to add the software firewall back in, but for now just turn it off.
as root in a shell do:
# service shorewall stop
to perminately turn it off type in drakxservices and uncheck the on boot button for shorewall. You can also start and stop the firewall from there.
Now you should be able to put in
http://64.x.x.x or whatever it was. This should take you to your linksys router then back in. If this works, it was the local firewall. If not it is probable a problem with the linksys not forwarding.
Keep us posted on your progress.
By the way, is your ISP IP static or dynamic?