Just to clear up a little confusion on the CGA card and boot process (sorry for dragging up a months old thread):
The CGA cards contain a ROM but it is only a character ROM for text mode - no program code is ever executed from it, and it's not accessible at all from software. If you have MBC-55x hardware but no boot disk specific to it, the best you will get the machine to do is emit garbage to the screen on an attempted boot. There's no BIOS logo or anything other than a blank screen output (on the onboard video) and a persistent attempt to read the boot sector on drive A. Before the OS has loaded from floppy, the CGA is probably outputting a signal with bad sync as the CRTC chip hasn't been configured at all.
A missing motherboard ROM, as on the pictures in this thread, however is fatal. (It is possible though for an expansion card to override the system ROM but I'm not aware of this ever being done. The hard drive option could possibly use this mechanism to allow booting from hard drive.)
As far as I know, here is the compatibility list for different RAM-BIOS/DOS disks (the hardware compatibility is entirely determined by the boot disk):
Early onboard video version: Runs on motherboard graphics, might operate with CGA card present with no RAM expansion on the card (won't use the CGA for BIOS screen functions). Won't boot at all with RAM expansion on CGA card.
Later onboard video version: Runs using onboard video, can utilize expansion RAM on a CGA card or probably some other RAM expander board. Make sure to fill out the RAM sockets on the motherboard first if using RAM expansion.
Early CGA video version: Operates CGA but doesn't work with RAM expansion on CGA card (not all the CGA cards shipped with expansion RAM functionality).
Later CGA video version: Operates a CGA card with or without expansion RAM. Again make sure the M/B RAM sockets are filled to utilize expansion RAM on a CGA card.
Hard drive version: (???) I've never seen the hardware or boot disks for a hard drive option, if anyone has this it'd be quite a find.
CGA boot disk & onboard video note: Onboard video can still be used through direct hardware access when booted with a CGA disk but the BIOS makes no attempt to use it. The boot disk displays a badly worded message in the corner of the onboard output, and eventually when a program uses the memory area, the display gets garbled. Of note is that the 32KB of dedicated video RAM (@ addr F0000) is not used by anything and free for whatever purpose you can come up with. I'm actually a little surprised they didn't try to load the BIOS/DOS there, it would have provided a little more RAM on a system that can be sorely lacking without an expansion.
Onboard video places the 640x200 green plane within the upper 16KB of any 32KB block of the first 256KB of memory (so at 16-32KB, 48-64KB, 80-96KB, etc up to 240-256KB). I suspect that this arrangement is why the "early" disks can't operate with >256KB, perhaps the code that decides this video RAM mapping screws up and crashes.
The red and blue planes (or for monochrome output, dim intensity plane and blink plane if enabled via dip-switches) are fixed starting at address F0000 and ending at F7FFF. Maybe the only reason for those DIP switches is to allow loading other data into the video RAM without affecting monochrome display.
Random MBC-55x trivia: RAM refresh is ensured by the onboard video hardware. If you set a very short output frame - say, only outputting a few bytes of video memory per frame, and execute a tight delay loop, after some seconds memory corruption will set in. Upon exiting the delay loop, chaos awaits. The video hardware must read at least 256 bytes per frame to avoid this - each memory read refreshes that corresponding byte within each 256 bytes of RAM. Anyone feel like coding up a true hardware random number generator?