http://www.crossroadsfencing.com/BobuinoRev17/#Mini
See if the mini-2560 has 3.3V option. If not, Bob would probably sell you one set up for 3.3V.
We're assembling 2560 cards now, all IO pins broken out. Just add power, or connect a USB Serial adapter
(FTDI Basic clone shown) and you're on your way. Shipped with headers shown; add IO headers
as you see fit, or contact us for other options. ICSP Header, DTR enable jumper, FTDI header with 5V enable jumper.
$46 plus shipping & paypal fee.
If you only need an extra serial port, more pins and RAM, look into the ATmega1284P, it's got 16K RAM.