If you want to measure throughput then close every other application that may possibly be accesing net. (Some access it openly and some intenally.)

Once that is done, retrieve a small file of known size from a fast ftp server(Ex. ftp.cdrom.com)

Measure the time to download this file. Then devide the size by time taken. That will give you the throughput.

Alternatively try Bandwidth meter by Cnet.

http://webservices.cnet.com/Bandwidth/

It offers to do the same thing I have described. Doing it yourself will be more accurate IMHO.