You could forward port 80 on the router to your SME Server, but in server-only mode certain important security features are not enabled. It would be better if you were to either:
1. Put another cheapo NIC in the SME Server, do not use the LAN side, configure in server-gateway mode. Configure for static IP on the WAN side and forward port 80 on the router to the SME server's IP. Use PPTP VPN to connect to your SME Server as if from "outside" and administer.
2. Again, put a second NIC in the SME box and feconfigure your network as follows:
Cablemodem <--> SME Server WAN side :: SME Server LAN side <--> router <--> PCs.
You have several choices with that latter: you could configure your router as an access point only, if possible, or you could have it take its "WAN" IP from the SME server and not change anything else.