There is also the new Wuhan-Guangzhou express train. It's the fastest train in the world and just began service in December 2009.
It will get you from Guangzhou to Changsha in little more than 3 hours! It's also less expensive, and by the time you add travel time and security screening time - it may be faster than flying.
It should not be rainy in October. We didn't see much rain in October last year.