How long in advance can I buy 2 tickets on G trains from Beijing to Shanghai for the 12/10/2011?

The Ticket Centre at the Hotel where we will be staying seems uncertain of the availability of seats during the National Holiday period if we wait to buy the tickets when we arrive at the hotel on the 4/10/2011: should I panick or simply use TravelChineGuide and have the tickets delivered to the hotel ? Thank You