High Speed Train (5 hrs) from Shanghai to Beijing

We arrive in SH on Thursday, 29 August, and need to leave on Monday, 2 September to go to Beijing - would like to take the High Speed Train (5 hrs) at about 09:00 in the morning from SH to Beijing. Is is necessary to book now, or do you think can we book 2 x seats on arrival in SH on the 29th? (would it also be cheaper to book at the station on arrival?)