Looks almost identical to my network, but I use an LRP firewall instead of smoothwall. Smoothwall wasn't well known when I setup my network, or I might have used it.
Since Smoothwall can do DHCP and the like I would leave the router out of the network, unless of course you are just using it as a switch. Which it looks like your doing.
I have one server for :Apache, MySQL, Mailserver. Then I have another server for fileserver and also have it setup for LTSP. This works out quite well for me.
Since it looks almost exactly like my currently existing network, I would have to say that great minds think alike. Or something to that effect.
edit: just read your second post. I see no overly important reason for the mailserver to be seperate on such a small network. It's not like your worried about internal security.
edit2: Just thought of one more thing. I also run a caching DNS server on my first server. I point all my machines to it and it's great to have nice simple name resolution inside my network. Also, keeps me online when my cable service loses their DNS servers, although this has only happened once. Takes a little while to setup, but it was alot of fun to learn.