IMO, Sox fans will not come out in droves after a big signing. Casual fans will. We've seen what happenes when they come out (Ligue anyone...?) A big signing will get "real" Sox fans talking, and maybe out to a few more games.. but a 500K jump in attendance.. not likely. They were already fooled once by the Belle signing (lord how I sometimes wish that NEVER happened.. ) and I just don't think they trust the Sox to do anything more than "rent-a-player" for more than a season (i.e.. Colon)
Plain and simple, winning for more than 1 year will bring the "real" fans back.