I understand that if I were to install a ASA configured in transparent firewall mode, I do not need to reconfigure the gateways (or anything else) on client computers. The transparent firewall acts as a "bump in the wire".

My network is x.x.x.1 with a SNM of 255.255.255.192. All computers are configured with public static IP in this range and their gateway is x.x.x.1

How can I configure a cisco 1800 router such that I can drop it in and act as a "bump in the wire"?

IOS do not support transparent firewall mode.

However, appropriate configuration can achieve full firewall features without changing anything in the PCs.

That's why I recommended a reputable consultant, in this profession we understand customer requirements and can configure the router to best meet them, without having the end user proceeding as trial and error.

i think i found the answer at the link below. my setup is the same as their diagram, except i do not have a HSZ, i only have a DMZ. well, my DMZ really is the HSZ, and my LAN and WAN are on the same network. i am going to see how to modify this to fit my needs.

http://www.akadia.com/services/bridged_cisco_router.html#Integrated%20Bridging%20between%20Internet%20and%20DMZ

The thing is, you can have some interface bridged and router to address them using a single IP like in the example, but you cannot bridge ones on different sides of the firewall, or NAT.

This "googled" approach to get things done "let's try this out" is exactly what I was referring before, that most customer do not want, because is business impacting, and somewhat dangerous.

On the other hand, if it's ok with you, and your users/managers tolerate experimenting, nobody can argue with that.