Hi ice789,
It's hard to evaluated your overall chances without knowing more about your candidacy, but I would recommend that you take another shot at the GMAT. Think about it this way: Your GMAT score is one of the pieces of your application that you can still control (along with your essays and, to a minor extent, your letters of recommendation). Why not take a shot at improving it? I'd hate for you to apply to McCombs with a 640, not get in, and then be left wondering if you should have tried to take the GMAT again.
Of course, there's no guarantee that your score will go up (unless you go with Veritas Prep for your GMAT preparation... we have a score guarantee

). But, even if your score goes down, admissions officers truly do look at your highest score, so it's almost all upside!
Scott