I second that. Having your websites and email run on the server that you also use for firewall and routing causes a lot of problems, since your internal view of the network is different from the outside view.
If you have a laptop that needs to work occaisionally on other locations, you'll need to switch /etc/hosts and stuff like that. It gets worse with things like PDA's and smartphones that switch from local Wifi to 3G. Getting bind setup for split-views is also not for the faint of heart..
Just stick in some extra dedicated hardware (although you still run the risk of the complications I sketched here).