The ATMEL 168/328 will run on quite a range of voltages.
It will run at a range of voltages, but it will not program at all those voltages. Do not try to upload a new sketch with anything less than 7.5V at the DC input jack/9V pin, or 5V at the 5V pin. Also, be aware that anything less than these voltages will result in garbage analog input operation, since there is no stable/known voltage to use as a reference.
-j