Depends on what your running on the SME box. If it's acting as the gateway there is no need to open any ports (Other than port forwarding 3389 for RDP to the Win2003TS box).
If you have the SME box as a server only behind another gateway then you wll need to open the ports for those services you are offering. If your running a web server then port 80 will need to be forwarded, if not then don't. Most common port to forward to a SME box is port 25 for SMTP email (again assuming your using the SME box for email).
Many of us open port 22 for ssh remote access, again good practice to port translate this as well, ie forward port 222 -> 22 on the SME box. Make sure that you DO NOT allow password access to SSH, read the doc's on how to set up SSH public-private keys
http://wiki.contribs.org/SSH_Public-Private_Keys should you wish to do this.
If your only using the SME box as a file server then no need to forward any ports.
Plenty of info in the documentation on this site, really comes down to what your using the SME box for.