The only effect that setting has are these:
It tells WinMx what basic bandwidth settings to place you on if you choose Cable or better during installation.
It allows you to make a primary if you have Cable or better chosen.
So it wont effect your speeds by changing it, although if yours was set to unknown at the time of installing WinMx it may be a good idea to go over those and make some changes to better your speeds.
That will depend also on whether your primary or secondary on the Network.
If you had a T1-3 line you more then likely would know this, most of these (not all) are business accounts, more then likely your a DSL (phone line internet) or Cable Internet connection.