What ADSL modem? What stats does it give?
http://www.kitz.co.uk/adsl/frogstats.php will give an idea how to get the stats, copy/paste these back into another post and we can then make a more useful comment.
One quick possibility is that if the downstream connection speed in the router is 576Kbps and the upstream is 288Kbps, then you are on a fixed speed product and not an up to 8Meg product.