OS X as a router


I would like use on of my OS X boxes as a router but I would like to make it more configurable than just telling it to share my internet connection over a given interface. I need to be able to set up NATs to some of my servers which I understand could be done with IPFW? I would also like to run snort which I dont foresee as being a problem. A web interface to administer my router would also be nice but I guess I cant have it all. The other option I have is putting this particular box between my existing router and a switch but will that mess up my NATs? I think I should be able to point my network clients at the existing router and by just turning on internet sharing the os x will just forward the data to the router right? anybody have any experience with this kind of thing?
This is a time when the Terminal comes in handy... :)

You might want to play around with some of the config files using the Terminal. Personally, I wouldn't know how to do it nor have I attempted it. :p You might be able to set up what you need that way and bypass the GUI tools which don't give you a lot of the options you want (at least with the client version of OS X).