particles will do it just fine, give them a high transparency and you could even put them in in post.

One thing i noticed: shouldnt the middle stained glass window be the same brightness as the other two... if not brighter? at the moment it looks like there are two major sources of light (if it's sunlight there shoud obviously only be one!)

You say you are only using area lights... how come? you should use a directional light for the sun outside. And then use point lights and area lights (sparingly as the up the render times) to pick out details add shadows etc.

Also the scene is too dark. If there is such a huge volume of lught coming through that window then surely light would be bouncing around all over the place inside the room. So the celiing etc would all be brighter and there wouldnt be any really really black areas.

Hope this is helpful

Alan