This is my point o view:
To do a 51-PC that can run some 200K aplications at time with a lot of strong VGAs, MIDIs etc is th wrong aproach. Why? Because we usually use real PCs with a thousans of MFLOPs and thousands of thousands of kB of memory.

So what is my point of view:

No loose the particular characterist of 52' series (the harvard achitecture) select a device with a large built-in program memory to central CPU, in this memory you can fit somethink like an OS and four or five aplications (don't forget that C64 rom is 20k and spectrum's rom was 16k) then in external memory you will put all the data with some shared memory to other devices etc.
Don't forget that in this device if i want a 'word procesor' it will be closest to worpad than a word version 2003++ only simple and useful aplications. If someone want to change one oplication to other then he can done with IAP but i think this 52-computer will not be the "30 on a day aplication system"