A good way you are headed...
There are two linux machines on the router--a laptop (wanna buy it?) and a tower, and another Vista laptop. None are ever used for downloads, except for ubuntu and Windows updates.
The linux machines' updates, as I recall, have presented highly variable rates, from the 10's to the high hundreds, 800 to 1000. Don't know about the Windows updates, because the MS dialog does not show the dl rate.
However, as before, I don't think it is the network equipment due to the speedtest results--unless there is a protocol issue (slow on one type of dl, fast on another...e.g., http vs. ftp, or TCP vs. UDP).