My current monitor is really terrible. 21" and blacks look like complete... well garbage.
After doing some research I found out that I probably want it to be 24" and to have 120hz. I also like it to be 3D and as the title says would like to purchase it for under 400$ but less would probably be for the best. I have heard a lot about the 27" korean monitors such as catleap but they all seem to be 27". If they were 24" i would probably get one of those on ebay.
Also I do not know too much about how the 3D technology effects things.