Have a look
here - A600 with 10Gb HDD, Ethernet, ATX PSU, 68030/40Mhz, 32Mb RAM - and all somehow fitting in the original case :-)
But even he says that buying a 1200 is obviously better. Just read about how the accelerator fits on (there is no chip holder, so an upside-down chip holder fits over the top of the original CPU :-D