The answer to this quite frankly, is "That Depends".
Presently, full 56k connection speeds can only be achieved in laboratory
conditions. Your connection speeds will depend on the distance you
are from the telephone company's Central Office (the further you
are, the lower the speed).
Your connection speed will also depend on the line quality of your
connection. Each time you place a telephone call, it will take a
different route to the person you are dialing. The call is routed
through different lines and switches, depending on their availability.
Some switches and lines are older, and therefore somewhat more corroded.
This corrosion will cause some slight noise on the line. Noise that
can create a slight annoyance for voice calls, can spell death or
lower speeds for Internet data connections. Therefore, on one Internet
connection you may establish a 43.6k speed, and if you hang up and
dial again, you may establish a 45k or better connection.
Line noise can also be introduced by the quality of your home's
internal phone wiring. The wiring in phone jacks can be loose or
corroded, especially in older homes. Also, the line coming from
your phone jack into your modem will need to be in good condition
for the higher speed connections.
Your actual download speed will be affected by the speed of your
computer. This is especially true if you have a software modem that
uses your CPU chip to process the modem data, instead of a dedicated
chip on the modem board.
Finally, your download speed will be a factor of the overall traffic
on the Internet. Also, if a particular web site's server is slow,
and/or there's a large volume of traffic on that server, your download
speeds will be correspondingly slower.