The problem is much of your stuff will be pretty useless on the enemies in Old World Blues at least it seemed that way to me. Old World Blues is also harder than anything in the base game but not as much of a challenge as Dead Money since you get to take your stuff.
To access it you need to be in the New Vegas area and you will probably be able to do it by the time you reach that area. Honest Hearts just feels like an expansion of the base game areas in terms of difficulty. I would save it towards the end but you can try it earlier if you want a challenge. Dead Money is much harder than anything in the base game and it strips you of all your items in it.
