Download speed

How can I accurately test my download speed? My cable company was supposed to double it from 2 to 4 Mbps. When I test at their site I usually get between 3400 and 3600 kbps which is good enough for me. However when I test at other sites I often get much lower readings in the 1500 to 2400 kbps range. One of the sites that gives me lower readings is a site their service techs use (http://performance.toast.net/). The cable co. says their own site is the most accurate but I am not sure I believe them......