cranky,
I use both IPCop and SME to serve and protect a Windows network with 4 sites. IPCop is a pure firewall & IPSEC router. It works well and is easy to setup. I use SME for services such as spam filtering, filtering email for viruses, http & ftp server.
Dividing networking/routing/firewall from services allows me to keep it simple and take advantage of the features of both IPCop and SME.
If you only have a single static internet IP, you can put SME on IPCop's DMZ, then use port forwarding on IPCop to pass necessary traffic to SME. This setup provides 2 gateways to your LAN, so you have to figure out a plan for the clients..and both SME and IPCop are a DHCP server, so you have to do some planning.
good luck,
ryan