Go ahead and build it now unless you're satisfied with your system right now and won't mind a couple more months.. Waiting is stupid since there will always be something new just around the corner.
I probably will. I'm taking summer classes right now though and I don't really need a computer that bad, so that's mainly why I was considering waiting for a drop on the dual cores.
I was also moderately worried about buying Vista right now, and still haven't decided whether or not I should stick with XP for a couple months when I buy this desktop.