If you don't want to pay the Mega price, here's a board that has as much as the mega, but half the price/size, and has FULL speed USB! (can act as a mouse/keyboard as well)
It's the Teensy, Arduino compatible for the most part. (I didn't see any i2c/spi examples, so not sure how that works!)
http://www.pjrc.com/store/teensypp_pins.html