What are the earliest areas that will still provide decent XP for a level 45?

I'm currently having some problems with Belial on Nightmare, and I've decided I need to farm a bit to gain another level before tackling him again (and either get some new upgrades to drop, or farm enough gold while leveling to buy something good off the AH).

I'm looking to clear up some achievements while I'm doing it, so I'm considering farming some of the earliest areas of Act II, or possibly even late Act I, until I gain my level.

However, I would like to ensure that the monsters I am killing are the appropriate level for my character, and avoid going too far back and fighting mobs that aren't earning me some decent XP.

What is the earliest area that I can fight monsters as a level 45 while still making progress towards level 46?

Solution 1:

Hit the 'Public Games' button in the main menu in order to look at which quests are selectable.

When you select a public quest, the description contains a level range. And if Blizzard recommends an area, I bet your character will still get experience in that area.