Motherboard or System is the closest you can get to seeing the NB temps. Easytune will also show you the system temps. Here is a post where Kick, a member here, tested the NB temp vs "System/Motherboard" Temp readouts
EP45-UD3 Northbridge information
As for your memory voltage question standard is 1.8V +/-
You can check PC Health section of the BIOS to see what you get for each setting, choose a setting and save/apply/reboot F10 back to the BIOS and check that page and you will see what actual voltage you get for each choice.
Some boards and or BIOS's will vary a little from setting to setting but the normal/auto should be around 1.8V.
Auto/Normal = 1.8V
+0.1V = 1.9V
+0.2V = 2.0V
+0.3V = 2.1V and so on.
If your ram is rated to 1.8V on the sticker or spec page then you should use 1.8V, but yes 1.9V-2.0V should be safe as well
Those NB/SB voltages you see in everest are just the stock spec of the chips, not what is being applied if you have changed the voltages. Everest will always show that amount though so no way to check the Voltages for north/south bridge.