I believe it's Asynchronis DSL. Think it's a faster version of DSL that businesses use.
If you're in the country though, satellite may be the best bet. It should be fast enough for Xbox live, but I would expect some issues now and then on connectivity.
I would stick with 56k but I do alot of file downloading and with the crappy connection that i ALWAYS get it takes most of the time an hour or more with alot of the files i download