Depends where you are downloading from .. normally an upgrade from 256 DSL to 512 DSL you should see an increase in speed. If its reporting 1.5mbps that would suggest you are on 1500, and you should see a HUGE increase.
Have you contacted your ISP ? Who are you with ? I understand there is a cable cut somewhere between AU and the US which may affect speeds to overseas sites.
EDIT : you shouldnt have to change any settings. On 512 DSL you should expect a max download speed of about 55kb/sec.