I have the ability to increase the download speed via my router ... I use custom Firmware on my Netgear DG834GT ...
DGTeam - The Custom DG834XX Series Router Firmware Web Site
This only works on the download speed and not the upload as the upload is controlled by my ISP. It is handy if I want the extra download speed but I have to be careful not to change the settings to often within a day as this can be detected as a fault when the router reboots to set the new settings. This can cause my connection to retrain for 10 days at a lower speed if it is deemed a fault. Within my routers config page I have a slider that allows me to increase or decrease the Signal to Noise margin on the line. The standard setting depends on the quality of the line and the SNR is normally set between 6 and 9 for a stable connection. As my line is fairly new and very stable I can lower my SNR to 3 and under. The lowest I had it set on was 0.8 with no dropouts on my connection for 27 days ..

The average gain in speed is 2 to 3 meg for me depending on weather conditions, radio interference etc...