Atmega128 is equipped with internal 4Kbytes of SRAM memory. Is it enough? Well it depends on what project it’s gonna hold. If your project must deal with loads of data or run larger RTOS code you will definitely run out of RAM pretty soon. Atmega128 microcontroller has an external memory interface built in, which allows to expand RAM up to 64 Kbytes.
Adding external memory to Atmega128 – [Link]