Speaker selection for difficult room conditions

I am looking to select a system that can double as both good 2 channel, as well as 5.1. The problem is that the room is a 2 story great room and is 15.7' x 18' and substantially open on one side (through an archway).

What Speaker Configuration should I be looking for? Are floorstanding mains a must for adquate full range extension?

I listen to classical, jass and occasional rock. However to be a credible home theater I suppose the speakers will have to be full range. My budget is around $4K. Any help would be welcomed.