There are only exactly 2 ways of getting more than 2MB chip:
a) emulate it, problem is, even a A4000-CSPPC+Voodoo3 running MorphOS or an OS4-beta will hardly reach A500-speeds(as you would need to run a full version of UAE).
b) create your own uber-Agnus/Alice out of a programable chip, remove the onboard Agnus/Alice, place a piggyback-PCB with the new chip and 6 extra MB into the free space. This might work, this might also end in the need of recreating Denise,Gary and Paula too.
None of them makes really sense.
But since I'm in the mood:
c) use a PC + UAE and somehow hack a PCI_2_Zorro-adaptor and a VGA_2_VideoSlot-adaptor. Now that sounds like a fun project, and should even be able to the toaster. But then just buying a the PC-Toaster might be easier.
