You might want to consider the STM32F103 -- they're programmable over serial in much the same way that traditional ATMEGA chips are, are arduino-compatible, can be found for roughly the same price as the 1284p, are a ton faster, and most versions have a ton more memory and flash.
You can see the full lineup of chips here: http://www.st.com/en/microcontrollers/stm32f103.html?querycriteria=productId=LN1565 -- I've personally used the STM32F103CB and STM32F103RE in a handful of projects lately.