This is what I'm doing with my '54GS. The little gateway that my ISP gave me (Verizon DSL) had to be switched to "bridge mode" which means that after activating the account, you enter the ISP gateway at 192.168.x.x and look for a way to turn it into "bridge mode"... which turns it into simply a set of blinking lights. Then the Linksys does all the work. I did, however, have to leave the ISP gateway between the phone jack and the Linksys... the net connection won't work without it.
I found instructions on how to do this to my ISP gateway on my ISP's website.