d20/pathfinder, is the rule set I am talking about using. The same rules engine behind D&D, and the newest Star Wars RPG, is the same engine used in the Stargate SG-1 RPG and D20 Modern. The difference with D20 modern, is that it's extremely modular, you only need to play with the rules that fit the setting, and it doesn't unbalance the game to do so. (Like removing magic from a D&D game, dramatically reduces the life expectancy of any party member for example) The other advantage of D20 modern, is that even the classes are designed to be modular, giving selectable talents, and feats instead of a rigid progression, it's also designed to REQUIRE you to multi-class, since no class in D20 modern has more than 10 levels. Then there are advanced classes, for building more of a specialist.