Steven,
I am not a linux command line guru, but I have used SME since version 4.1.2. If you have another low end computer available, say a PIII with 128mb of RAM and 3 nic cards, you can install IPCop firewall. If you have only a single internet IP to work with, you can setup IPCop on your external internet IP. SME would then connect it's external nic to the DMZ subnet of IPCop..(I use 172.16.x.x for DMZ). This will result in two available gateways on your internal lan...one for SME and one for IPCop...your clients will use which ever is the dhcp server assigns by default.
IPCop is easy to install and has an excellent web admin interface. It is only a router, and does not have apache, samba, or email, so you can port forward any port to SME on the DMZ or any host on your internal network. Just remember if IPCop forwards to an internal client, that client must use IPCop as its gateway. Since your wanting to port forward 80 to an IIS web server which should have a static IP address, this is not a problem.
IPCop also has IPSEC VPN built in that is easy to use. I even have remote locations that use a $130 Linksys VPN router to connect the IPSEC tunnel to IPCop. I personally have found SME addon features (IPSEC & Port Forwarding) problematic when attempting to upgrade SME, so I keep SME default and rely on IPCop for true router functions.
Good luck,
RS