Circle line is necessary as it cut short the distance people need to travel say from Yishun to Tuas and to Punggol etc and vice versa. On the other hand, the rumoured another East line through Marine Parade is not really necessary since there is already a East-West line and people can switch to bus from nearest MRT station to go there.
T relieve jams in CBD, the right way should be to make the MRT stations in CBD walking distance to each other (<=500m apart) and get ride of all buses in there (they are causing so much jams over there and also because of reserving 1 stupid bus lane!).
Make it just like in London central where each Metro station is <=500m apart and hence taking bus is not necessary. Make sure the station is wheelchair friendly as well.