Knowing your broadband speed is very helpful too, as well as the ability to use VoIP.
I tend to use two speed tests, as I believe that using the same reference test sites makes it easier to determine just what kind of experience you will have:
Cox Speed Test can be found at http://test.lvcm.com/
Visualware’s VoIP Speed Test which is located at http://myvoipspeed.visualware.com/