I just got ADSL hooked up today, 512kbps.
I am downloading a few files such as modzilla from download.com and a few others from some other sites, and the fastest speed I have got today was 20kbps.
To try and further help you Longreach, you mention that you were downloading at 20kb/sec. That would be more typically seen on a connection that is ~160Kbps; not 512.
Under optimum conditions, (which, as Whitehatter points out, depend on your location from the server) you can expect your 512 Kbps connection to download at about 60-65kb/sec.
You might want to have your ISP
come out and do a signal strength test. In some cases, DSL subscribers find out (after they sign up) that the signal in their particular neighbourhood is too weak to provide decent speed. Other factors though, as simple as line interference from an old phone in the house, can degrade the DSL.
Good luck, and if you have any other questions, feel free to e-mail me.