"secure enough" is a relative term. For many small businesses, SBS configured properly with ISA Server is "Good Enough" security. (see this article about that: http://sbsurl.com/security).
Personally, I think that in most cases, ISA is fine on it's own, but I always put a hardware router/firewall in between SBS and the Internet connection. Usually nothing more than an inexpensive router/firewall which provides stateful packet inspection... something along the lines of a NetGear FVS318.
If you already have implemented Outlook Web Access or Remote Web Workplace (http://sbsurl.com/rww) then you really aren't exposing your server any more than you are currently because RPC/HTTPS uses the same port 443 that's already open for those services.
Jeffrey B. Kane [SBS MVP]
TechSoEasy
http://www.techsoeasy.com
http://techsoeasy.spaces.live.com