There's ALWAYS something better just around the corner. It depends on how much you want to spend, how often you want to spend, and how slow your current machine it.
For me, I will not upgrade unless the upgrade is more than twice the overclocked speed of the current machine, and costs what I want to pay. IE if I have a 1.6A, overclocked to 2.3, then my next upgrade will not be until 4.6GHz, at a price that I feel is OK.
If your current system is nice, does everything you want, I suggest you hold off until the Hammer, or perhaps even the first revision of the Hammer.