No after you go to act 3, you cant go back to act2, unless you load a save from act 2...
I wish i could help with the armour sets, but i have never been really interested in them, i find the tranquility / serenity sets worth my while, but none of the others have really caught my eye yet <img src="/ubbthreads/images/graemlins/disagree.gif" alt="" />
