if you're having problems with connection speed...it's probably a phone line problem.
If you have the capability...I'd run a dedicated cat5 line directly from the computer to the telephone interface box.This may help.
As far as dialup ISP's...I tried several(before we got DSL)...and Netzero3G is by far the best I've come across...also the fastest dialup connection I've seen as of yet.(not a bad price either)