Hi Elia,

for such applications I use a separate SRAM module.
On my web you can found the schematic for a 512kB SRAM module. It support sequential access (automatic address increment) and hold data during power down (several month with an 1F Goldcap).

http://www.specs.de/~danni/appl/hard/sram/sram.htm


Also the Atmel data Flash series can be interesting for you (up to 4MB).
There is an audio recorder application note with it on the Atmel web.


Peter