G and D trains

We will visit Hangzhou and Shanghai for a few days, arriving on 21 October, 2011 in Hangzhou. Will that period be a busy tourist season or is it okay? We plan to take the G or D trains to get to Shanghai. Hope that the train tick availability is good. Also, when I checked the train schedule, all the trains numbers for Hangzhou-Shanghai are prefixed "G", but when I checked the travel time, some trains are with 1.5 hours or more, so these are actually the "D" trains, although the train numbers bear prefix "G", is that right?