I found this site to be pretty good at explaining much of this:
http://www.pcpitstop.com/internet/bandwidth_about.asp
The site includes Bandwith/Internet Connection tests one can run, with a thorough explanation of the results (along with many other tests for overall computer health - one must register, however).
From the site:
"The connection speed between the computer and its modem (called the Maximum speed under Control Panel/Modem/General tab/Properties) should be set as high as possible without causing errors. On most computers this is 115200, also written as 115.2 Kb/s.
The connection speed between your modem and the ISP’s, and the compression and error checking, are negotiated between the two modems when they establish the call. In the very best possible case, which is rarely seen, two V.90 (56 Kb/s) modems will be able to connect at 53 Kb/s with compression, and the compression on normal text transfer will average 50%, giving an effective transmission rate of 106 Kb/s. Very highly compressible material could be transferred at the maximum rate of 115.2 Kb/s. Incompressible material like ZIP files could be transferred at a maximum rate of 53 Kb/s."