You can daisy chain routers (almost) all you want... The wrt thing is actually both a wap (wireless access point) and a router in the same box, sharing power/a wall wart.... no such thing as a technical difference between n and g routers... If a salesman told you any different and his lips were moving, he was probably lying (or training to be a politician :)
as for g or gs doesn't really matter, both work about the same for what you want to do, just the g model (no s on the end) is cheaper (at walmart g is
48.88, gs is 78.96) up to you if you want to spend a few bucks more.when you do the configuration on your new thing (or if you reset it cuz you messed up), you can connect to it either wired or wireless, and in your browser go to 192.168.1.1.... I would suggest use a cable for the initial config and turn off wireless, you can also set the starting ip address for the whole enchilada (it starts/resets to 192.168.1.1, since you already have one device at that address i would set the second at 192.168.1.200 ), and if you want you can also change the starting address for dhcp etc (don't mess with that), Comes as a default of 4 addresses for the router part, and default dhcp server provides 50 more.... oddly enuf, that adds up to the max number of 254! (that's why I use 200, easy to remember and works out real easy)