160bytes @ 50Hz is only 64kbits/s (average). Even SHIFTOUT should be able to handle that.
Running the SPI or MSSP module at a moderate 1Mbit/s will transfer those 160bytes in less than 2ms leaving 18ms for processing between frames.

Not sure why you're even considering external RAM.
Even a double buffer should easily fit an 18F series part unless you're using a lot of RAM for other tasks of course.

Also not sure what part ARRAYREAD plays. Transfering the buffer would be done using a normal loop or, as Jerson mentions, DMA if you want to go the extra mile.

/Henrik.