It all depends on how many bits your SupraDrive needs. If it's 16-bits, you'll need 4 of those x4 bits to get a 16-bit word size. The number of words will also be multiplied. So if you get 4 256x4 DRAMs that will be 512K of RAM. If you get 4 of those 1Mx4 you'll get 8M of RAM.
-edit-
Going with the example you gave of wanting 1 Meg of RAM, you'd need 8 256Kx4 chips.
