If you're on maxdsl the following table will be relevant to you.
Whatever speed you are synced at will determine what IP profile you will be on.. so for example if you're synced at 3904 Kbps then your IP profile will be 3000 which would mean your download speed would be about 370KB/sec.
You cannot increase your sync rate as this is how fast your line can handle, however sometimes rebooting your router can increase the sync ever so slightly.
You can also find out your IP profile and do a speedtest via here but you will need your ADSL login.
What is an IP profile? well it's basically a speed set at the exchange according to your sync. If you live closer to the exchange then obviously you're going to get better speeds (have a higher IP profile).