They is basically impossible to predict. Put it this way - the bubble will continue blowing until it just pops. There is no way in telling when it will pop, you just have to keep looking around.
Some of the prices that are being quoted for llll.coms look crazy. Especially when the letters make no sense together. But it is the brandability factor I guess. You can always make a four letter, even if it is utter nonsense, stand for something. As for valuation, depends on demand really.