ElectronicsNoobie:
As per title do I need a voltage regulator to build a standalone arduino circuit or can I use a voltage divider as the required voltage of the ATMEGA328 is 1.8V to 5.5V according to http://uk.farnell.com/jsp/search/productdetail.jsp?SKU=1848694.I did post a related topic in the product guidance forum but as I haven't received any response I decided to re-post here as this forum seems more relevant.
You don't need to use a voltage regulator, but it is on you to be sure voltage can not exceed 5.5vdc or you risk chip damage. Don't try and power a chip with a voltage divider, it not the proper way to provide power to a chip.
Lefty