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.
Bob, I've finally gotten around to populating the 1284 Duemilanove board you sold me. Still waiting on the 1284p chip but it's checking out with a 644P chip I had on hand. I really think if you or someone offered this in a completed assembled package with a included CP2102 cable that it could be a good seller.
Have you had any problems yet with the software mods needed for the IDE? Any libraries not work? I've only run blink and a few other simple sketches just to verify basic operation so far.
Only question I have is the intended purpose of the 6 pin header in the corner of the board. Not real clear to me from the schematic what pins it actually breaks out? So far I didn't put header pins on it.
Lefty