We don't know when the 6 core gulftown will arrive, or the price.
I suggest you build with the 920 now and see how it goes. Most likely, you will be happy, particularly if you can give it a safe overclock to 3.2 or so.
If you think you need more cores then, replace it with the new cpu. You will not take much of a loss on the 920 reselling it at that time.