Keller Williams Gold Coast is the latest of Chicago's "Big Boys" real estate firms to go by the wayside.  Today, the firm and its 200 agents agreed to be acquired by Coldwell Banker Residential Brokerage.  Firms that survived in good times on high volume sales are finding their business models are underwhelming in a lean sales market.  Gone are the likes of Sussex & Reilly, Sudler Sotheby's, Jameson (Now a mere shadow of itself after taking on the matel of Sotheby's International - and most recently dealing with the embarrassment of having one of its principals, Michael Sato, charged with shoplifting ties from Bloomingdales).   Who is next?  Who would you rather do business with?