How far can a desktop A1200 be expanded?

[Boot_WB]

The Ateobus, like the mediator, would require your amiga to be towered.  The bus itself is a 4-slot ISA bus into which cards (pixel64 graphics, scsi, high speed serial/parallel, ethernet) could be added.
Pretty good actually, aside from the drivers requiring keyfiles to run them.  PAin in the ar5e now that ateo concepts (the french company who made them) no longer exists.

[Boot_WB]

Maybe if the a1200 was installed into a PC desktop case, but not in a classic 1200 desktop case.
It's pretty much the same space requirement as the mediator, with the difference that the busboard is connected to the expansion port pass-through by a couple of ribbon cables.  The upside is it allows you a little extra flexibility in where to mount the busboard.  Quite handy if you're adapting a tower, rather than installing into a custom-built job.

I've got one of these, and iirc it does 1024x768x15 nicely and 800x600x24.  Can't remember exactly, as have installed a mediator SX since, and haven't played with the Ateo for a while.  Some graphics corruption in the menu bars, but I assume that was my inexperienced installation of Picasso96.

You're right, the Pixel 64 is basically a cheap ISA graphics card, but it does the job nicely.  Was probably quite an expensive card back in the day.