There is also the progam bitlash, where you run an intepreter on the '328 and it runs code that is on SD.http://bitlash.net/
Bill Roy contributes here regularly.
The 1284P is a very nice chip, lots of IO, and designed to be compatible with shields.
(Serial is still D0/D1, SPI is still D10-11-12-13, I2C can be A4/A5 with a jumper setting, or they can be on the "new" pins and A4/A5 can still act as A4/A5 or D18/D19. PWM ended up on D7-8-9-10-30-31, so you can still have 6 PWM and not lose SPI capability. And the chip supports 2 hardware serial ports (Serial, Serial1, and 3 hardware interrupts, INT0, 1,2).
Add the mighty1284 core files from maniacbug, and I can post an updated pins_arduino.h file that corrects a couple things. (I should put that info with the board link! Will remember one of these nights, vs posting it here over and over).https://github.com/maniacbug/mighty-1284p
Onboard USB support with mikroe483 from mouser.com, or offboard with an FTDI Basic or equivalent if your embedded project doesn't USB support after test/programming.http://www.crossroadsfencing.com/BobuinoRev17/
for schematics & suggested PL