Add Memory to the Arduino ADK

Hello everyone,

I need some information on how to add external memory to the Arduino Mega ADK that has 256 Kbytes of flash memory. This is not enough for my project. So which chip does fit to be interfaced with the Arduino ADK for more memory?

All of your help is of appreciation.
Thank you!

See the memory modules at www.ruggedcircuits.com that fit onto the double headers of the Mega.