both bus and trian are OK, and both of them need transfers. I prefer those D-trains, they are fast and save time.

They are from Sushou Railway station to Shanghai Hongqiao Railway station, after getting of the train, you can take subway line 2 to People's Square station, then walk to Nanjing road.

Thanks for your reply =) So how much does it cost and how long does it take?

It takes 30 minutes or so by D trains from Suzhou to Shanghai Hongqiao, costs:26/32RMB. So, it should be no more than 40 indluding the subway fare. There are also G trains which is a bit faster than the D trains, but more expensive, 41-48RMB per person. However, the G trains can take you to Shanghai Railway Station which is much closer to People's Square and East Nanjing Road by Subway, only three stops by Subway Line 1.