Correct me if Im wrong but, the ATMega8 and ATMega328 are identical pinouts, so if you use an eagle footprint for a mega 8 it should be able to use a 328 chip interchangeably, barring other differences like mem size and so on, from an arduino pin out point.
Yes, you are correct, at least in the DIP package. Never seen the other ones.
Yep; I swap CPU types in my arduino boards "frequently." The ATmega168 is also pin-compatible and supported by the Arduino environment. ATmega88 and ATmega48 are pin-compatible but not currently supported by the environment.
Thanks bub and westfw, I had one of my early eagle designs, done way before I understood more than I do now, GRIN, and wanted to make sure I wasn't missing something hidden or commonly known or if had to change out the mega 8 for a 328 design.