Barebones 328p, 2xAA vs 3xAA, which will last longer?
Yes, running at a higher voltage will consume more current.  In the datasheet you have this nice graph:



I have highlighted the two voltages at 8MHz for you. At 4.5V you would typically be using about 4.7mA.  At 3V you would be using around 2.8mA.  That's not taking into account anything else in your circuit of course - just the ATMega itself.