technically your N router should run G but it doesn't matter.
I did not mean to imply that you would connect them via wireless...you want to connect them via wired.
You would have to buy a switch to place between the modem and the routers, the big question would be if the ISP has locked it down to a single device.
If we think of it another way. Say you had a router (with no wireless) and 2 AP a G and a N. You would cable the router to the modem and then plug the APs in the LAN ports of the router. You have in effect now built a router with both a N and a G radio.
But if you use a wireless router, one of your AP are already active in the same physical device so you now only need 1 extra AP which you plug into the LAN port.