I use the '1284P a lot, bigger memory version of the 644. 16K SRAM, twice that of the '2560, and much lower cost - like $7 in qty vs $17. 32 IO.
Easy to set up the IDE so it can just be treated like another board.
http://www.crossroadsfencing.com/BobuinoRev17/
Poke around here.