Looking good Nir.
ok I found out about dirt maps for you. There not necessarily to make your seen look dirty or old. The dirt map shader is also used to fake occlusion. FG produces a occlusion affect. But with GI or allot of lights like in a seen you have hear. The occlusion is obscured some what leading to not very well defined lines along edges. It also allows you to lower the FG setting father because it takes care of the corners more. Instead of turning it up to get the grain or artifacts out of the corners.