Have you considered just running everything on 3.3V? I designed a data logger that runs on 2xAA cells and uses an MCP1640 boost regulator, that when not enabled, simply passes the battery voltage straight through. The logging cycle is: Wake up (via RTC interrupt), enable boost regulator, increase system clock speed to 8MHz, power sensors on, take readings, save data, program next wake time, power sensors off, reduce clock to 1MHz, disable boost regulator, go to sleep. Using 1MHz allows the MCU to operate down to 1.8V when running on battery voltage, but while reading sensors and saving data, everything runs on the regulated voltage.The regulator can be configured to supply either 3.3V or 5V; exclusive of sensor requirements, everything else on the board is equally happy with either but of course 3.3V will provide better battery life. Sleep current is only a few microamps. It uses EEPROM for logging memory (hence the 512kB limitation). I've thought about redesigning it with an SD card but that's a low priority project.
I had heard it said that 1MHz was the sweet spot for low power
I'm really happy with the way the data logger works and the power consumption looks like it probably exceeded my expectations by a fair amount. The logging cycle can be as short as 15ms depending on sensors. Just received data from a couple loggers that were in Northern Michigan through the winter. The first was inside an insulated but unheated cabin where temperatures got down to 1°F. Its regulator was configured for 3.3V. It started with a less-than-fresh pair of AA cells, that measured 3003mV at the start, and 2987mV after 184 days. The second logger was in an open shed. It used 5V, and its battery started at 3255mV and ended at 2844mV after 195 days with temperatures as low as -22°F. It had more sensors and more complex sensors than the first unit; I don't have current measurements but I imagine its sensors drew 2-4 times more current than those of the first unit, and for longer intervals.I can tell you get a kick out of the low power capabilities these MCUs have. So do I!