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