I would suggest becoming a good player at a wide variety of games, for several reasons. If you only know hold'em, and you get invited to a home game with very weak players that is going to be seven-stud, you'll have to pass. Or if you're in a mixed game, you are going to get killed by someone whose knowledge of all games is only average. Also, as said before, there's always a chance that hold'em will lose it's popularity. Finally, if you're in a casino looking for a game and there's a really tough hold'em game and a really weak omaha game, a player average at both games will be better off then a very good hold'em player.