If you truely think new baby will use them can you box them and store them in loft or garage?
If not you have to be ruthless. WE have no storage at all and no plans for more babies so we have been doing a sort out before Christmas too, of clothes and toys.
Toys have generally gone to DD's nursery. She still gets to use them if she wants there and the other children get different toys to choose from. Or you could send them to a charity shop or children's hostel type place.
Clothes we pass on to a colleague of DH's. It is the woman's daughter who has a baby about 6 months younger than DD. Apparantly they don't have much money and do struggle at times so feels like a worthwhile place for them to go.