the x3100 will be fine. in 3d it isnt even really that much slower than the GS. the only real advantage to the GS is if you have say a blue ray drive on that laptop.
the 3100 though will probably save conserable electricity.
the 965gm/pm are the same die so , bascially video is "free". so if you use a 8400gs , you still have a x3100 on the die of the 965pm just doing nothing.