A common misconception is that your DSL line rate is the speed you can download stuff at, it is not.
ISP's as previously mentioned use Rate Adaptive Technology to monitor your line and set what is called your IP PROFILE accordingly, that is the actual rate you can download at.
Your DSL might be 8mB but if your IP PROFILE is only 200K then thats the fastest you can download for example.
Normally your IP PROFILE is around 70% of your DSL line rate.
There have been many reports in the UK of the IP PROFILE not updating, so if your download speed is less than 70% of your DSL line rate I would certainly consider taking it up with your ISP.
So regardless of what your line rate is, the most important piece of information you need is what your IP PROFILE is, it is set by your ISP and you can check with them what it is.