As for rules, I've written a short (6 pages) mod for Legends of the Old West. Anybody interested can drop me a PM with an e-mail address and I'll send you a copy of the PDF.
Also, there is the very detailed and, in concept, rather sound "Fallout Warfare" ruleset. This is available at various websites as a (legal, IIRC) download, for example at http://fallout-rpg.narod.ru/Warfare.html
. Much more complex than my own approach, with lots of stats and detailed figure profiles worked out already, but primarily focused on Fallout and Fallout II. Also, it is "quasi-official", in that it originally was a "goodie" included in one of the PC games (I think it was Fallout II or Tactics) and written by members of the PC design team.