Hi Lucky009,
First off, a 640/Q49 is a strong score (it's just shy of the 80th percentile overall), so it might be enough to get you into McCombs. While it will be at the 'low end' of the Schools GMAT 'range', if you have a strong OVERALL application then you could still get an invite. That having been said, it's understandable why you might want to retest.
With a December application deadline, you have plenty of time to continue studying and work on your application(s). However, since your last GMAT was 2.5 months ago, I'm concerned about how much your GMAT skills may have 'faded' (especially if you've been studying 'off and on' since then).
When was the last time you took a FULL-LENGTH CAT (including the Essay and IR sections)? If it's been awhile, then you should start by taking one. Once you have those scores, you should report back here and we can talk through your options.
GMAT assassins aren't born, they're made,
Rich