Will Atmega328 (with 8mhz int osc) start with lowest charge of 2AA Duracell

Yes it can, see the speed grade chart in the datasheet.

You may need to set the Extended Fuse Byte for a lower Brown Out Detection level.

These are for a '2560, '328 has similar chart.

Speed Grade Atmega2560 and V.jpg960×720 39.3 KB

Speed Grade Atmega2560 and V.jpg960×720 39.3 KB