CrossRoads:
A good chunk of that is the Tx and Rx buffers for the serial data.If you need more SRAM, consider the '1284P - 16 Kbyte of SRAM, twice what the '2560 has. Dual serial ports, 32 IO lines.
I have a nice Duemilanove style board for the 366/644/1284 series of chips.Cross Roads Electronics
Can use onboard FTDI module, mikroe483 from mouser.com, or offboard with FTDI Cable, FTDI Basic, or similar CP2102 module.
My main project is actually using one of these chips (love em! the extra ram is awesome) but my issue was that I need to communicated to individual devices over longer range than I2C will allow without a booster chip and my cheapest solution that I could find was the ATTiny45-20PU chip (NeWark sells these at 62 cent a piece if you buy 25 or more at a time) and I was worried about the memory foot print. I guess I will just order one and run software serial on it (or attempt to run it... 250 bytes of memory) and see where I stand. All the ATTinyies will have to do is read things like sensor values and transmit them to the main board which if I can get software serial to work on it would be perfect solution for the project.