Regarding DSL, the max speed that you are given is mostly decided by the quality and length of the phone line you have. If you have a regular connection they will give you 768, and if they have trouble making that work because of line condition or radio frequency interference or bridge taps, they will drop the speed to 384. If they cannot keep a connection working at that speed they will just deny you service - the FCC will not let them offer you DSL service slower than that because of the way it was initially offered, even if you say it's ok and you'll make due with 380. Some providers will even go up to 1536 if the connection will support it, but that is rare. DSL works only out to a distance of 14,000 ft from the local telephone company Central Office (CO), and the closer you get to the limit the more interference the lines will pick up... the phone lines act like antennas and pick up all kinds of RFI interference. If you live within a mile of a radio station broadcast tower you will not be getting DSL. Period.
Some of the providers will offer DSL up to 6 meg speed, but the cost is usually out of the range of what most users are willing to put up with, and the other bottlenecks in the Internet usually cancel out the benefits. And again, it all depends on your distance from their central office.
One other thing you can look into if you live in an area with SBC phone service is to inquire about "project Pronto". This is where they run fiber optic lines out to a 'neighborhood substation' called a CEV, and the copper runs from there to your house. The 14,000 foot limit is measured from the CEV instead of the CO, basically expanding the coverage for DSL. It costs them a bundle to put one in (million plus), but if there are enough customers in an area that want the service, they will do it.
Based on what I learned working for one of the DSL companies, I would look into cable first... you can be installed within 24 hours, while DSL is about a 2 week miniumum - and I have seen delays as long as 6 months for some DSL customers.
-Chilehead