Just fast info.
There will come a new mega324 called mega324PB, this is a new design, and will have same problem than some new 328 (and similar), Atmel call it a 20 MHz chip, but there is NO way to run it faster than 16MHz unless you give it an external clk!!!!
And it only have a low power crystal swing. (so problems with industrial applications).
On the good side it will have 3 usart's and extra things here an there, but the normal warning, the datasheet are full of errors. (the one that made additions for sure don't know anything about the chip, why don't they have someone do proofreading before it become public.)