How fast is your cable internet connection running right now? To be honest, I don't think modifying the modem will really offer you any better download speeds.
As far as I know, the speed of the internet connection is regulated by the ISP rather than the modem itself. Also, even if you were to drastically increase the available bandwidth, single files and web sites will still run about the same speed due to bandwidth throttling on their end. So, I would resolve that even if you could get a decent speed increase, the actual usage of the hack would be well, mediocre.