as Kush said, the only reason why I got rid of my wrt54g at the time is because I went from 20mbits up to 50mbits (many moons ago) and I discovered that the most I was going to get wireless was 22mbits. That prompted my upgrade to the wrt150n at the time.
This is one of the rare occasions when sticking with the shub is actually going to be better for you. You are on the right track though, if you want to stick the shub in modem mode you can do, you'll just be better off with a more up to date router which handles 802.11n and dual band etc. Something like the EA2700 would be good for £50 from Amazon and I can guide you through how to set it up.
http://www.amazon.co.uk/Linksys-EA27...linksys+ea2700