They're the actors it's safe to bet your Oscar dollars on. But bet on them when they're playing characters who are mentally challenged, and you're sure to win the pot. Whether Cuba Gooding Jr. follows suit with his turn in ''Radio'' remains to be seen, but it's clear that such roles offer anything but limited opportunities. Just in the last two decades, nominations have gone to Leonardo DiCaprio for 1993's ''What's Eating Gilbert Grape;'' Robert De Niro for 1990's ''Awakenings;'' Billy Bob Thornton for 1996's ''Sling Blade;'' and Sean Penn for 2001's ''I Am Sam,'' in which he plays a father fighting for custody of his daughter. And then, of course, there are the winners: Dustin Hoffman, for 1988's ''Rain Man,'' and Tom Hanks, who became America's favorite chocolate aficionado with 1994's ''Forrest Gump.''