how to properly use external memory flash with MCU (dsPIC33)
Take this points in consideration before starting:


You must ensure stream speed you going to generate (how many
kBytes per second?).
I know people reached 700 kBytes per second
write speed on generic SD card on PIC MCU
http://www.microchip.com/forums/m626688.aspx. Maybe its enough for you.
Are you sure that
selected MCU can handle a stream you mentioned? 
Are going to
generate burst data (large amount in little time, then pause)?
if   Yes, then use RAM for buffering data, then write it to any slow storage.