From what I understood, every extension like uaegfx won't work with the PPC emulation since it requires the 68k (don't ask me why :)).

You can use a Picasso IV with PPC emulation. It is emulated by UAE (and limited to 4mb Vram).

You cannot have more than 128mb memory since there is no os4 driver for the cybppc scsi extension.

Tony said supporting more memory could easily be done but requires a new OS4 build for that, which is very unlikely.