Most of the country is not lit at night. There are a couple of temples that are really lit up - but for viewing at a distance, not for walking around.

Internetwiz, you may be underestimating the size of the area which the temples occupy. Many square miles. Old Bagan is in the center of the western edge, as I'm sure you've noted, with the other 2 towns north and south, but all are just at the edge, between the river and the plain. If you've found a place you like in Nyaung-U then I see no reason to change. In fact New Bagan & Nyaung-U have certain advantages like lots of choices of restaurants, the markets, shops to walk to, and which Old Bagan, as far as I could see, was lacking as they've been moved out. My preferences, as I mentioned before, were the 2 other towns and I stayed in both by choice.