I use a similar setup. I use Verizon DSL thru a Linksys firewall/router to a SME 6.0 Server to my internal network.
Linksys
Set the outside IP on Linksys to whatever your ISP says to use
Set its internal IP to 192.168.0.1
Set DNS to your ISPs advised DNS
Set Gateway to ISPs advised gateway
Port forward 80, 25, and 443 to 192.168.1.1
SME Server
Set your external SME 6.0 IP to 192.168.0.2/255.255.255.0
Set SME gateway to 192.168.0.2
Set SME internal IP to 192.168.1.1
Internal Network
Set workstations to 192.168.1.x and gateway to 192.168.1.1
DNS to 192.168.1.1
Run a shields up port scan at
http://www.grc.com to confirm your ports are visible. That should be all you need.