I know the wrong MTU setting will slow the internet. Running *:Windows racet.exe can tell you the MTU your System or ISP is running at.
In your modem settings if you enable compression for your modem your packets will be changed to what works best for that modem card. By useing the cable or DSL settings for windows. Lets the operating system get the packet to the card faster so the card can compress the packet. For most newwer cards the packet will not be higher then the hardwares rated size. The system needs to uncompress the packets for you to use it, making the transfer faster.
For some system this will work very well, If configured correctly.
It all boil's down to what works best for you! I did a lot of testing when I had a dial-up and the bit of info I learned worked very good for lots of systems.
To test your settings of how fast your system is configed, not form point to point! Here:
http://forums.speedguide.net:8117/ telles your MTU, MSS size and more.