Or take the easy way out and pick one of the many Untangle partners that know how to properly build the box up front and go buy a box from them.
http://www.untangle.com/Partners/Find
Some googling will yield even more options for you.
An Atom based solution is a bit light for your needs. If you didn't have all those VPN users, I'd say you could use one. However, with the CPU load of the VPN + the CPU load of the SPAM engine things could get entertaining. That doesn't mean it wouldn't work, it just means I'd be a bit gun shy about suggesting an atom.