speedtest.net is a good site, but it only tells you a little bit about your speed.
Many ISPs will ramp down the speed after the download starts.
The true way to see your speed is to download a large file from a server that you know is faster than your connection.
Download this:
http://download.microsoft.com/downlo...35-SP2-ENU.exe
Pay attention to the download speed. For example, my speed averaged 1.9MB/s. You multiply that number by 8 (8 bits in a byte) to get 15.2Mbps.
That number is a real world number that will show what your ISP is capable of.