RewriteEngine on

     RewriteRule /overview/ /cgi-bin/board.cgi?action=overview [R,NE]
     RewriteRule /board/([0-9]+)/(.*) /cgi-bin/board.cgi?action=viewboard;boardid=$1;$2 [R,NE]
     RewriteRule /thread/([0-9]+)/(.*) /cgi-bin/board.cgi?action=viewThread;threadid=$1;$2 [R,NE]