that even though they may "offer" that kind of bracketed service, you may not have the infra-structure in your area to handle the speed.
Remember, we are talking COPPER WIRE here which has a lot of resistance.
Unless they have put in a REMOTE TERMINAL ( RT ) in your area, I doubt that a regular copper wired phone line can give you 6,000 on the download.
Also, you are gonna have to be within 12,000 feet of the Telco switching office too, which I assume that you are if in fact you already are getting SBC DSL Service.