The parts follow the same basic memory architecture. The amount of memory on-board the part is different.

Original Intel 8051 - Code Rom = 4K
Newer Intel 87C51FA - Code EPROM = 8K

Original Intel 8051 - Internal RAM = 128
Newer Intel 87C51FA - Internal RAM = 256

Note also the 87C51FA has more onboard peripherals. In particular there is the
PCA and the T2 timer (which actually appeared on the Intel 8052).

Hope this helps.
Michael Karas