First, a download manager makes perfect sense for a user using dialup as IE's downloader doesnt have the option to resume downloads
Then, DAP is not a good download manager, it has adware, in other words software that pops annoying ads at you and tracks information about you. Better use Getright, Star Downloader or Flashget (My recommendation, but try out the others first if you have never used a download manager)
Lastly, no, you wont get 300% increase on dialup, the 300%, if im right, refers to those cases (at high speed broadband) in which the ISP imposes a speed limit on each individual download stream, and the download manager speeds things up by downloading the file in seperate streams. You may get a slight boost but not that much
So hope this helps