You can use a wzr-hp-g300nh router with cable or DSL modem either, so long as the modem isn't a USB only style modem (it has to have RJ-45 LAN port)
Also, you can do what you are wanting to do with no prblems, you may also find that you may not need another router, you may only need just one, I had the same problem in my house, i got 3 stories, the signal on the ground and 2nd floors were great, 5 bars all the time, just in my undergound lvl, where my computer lab is, the signal was only 3 bars at best, i bought 1 WZR-HP-G300NH and it covers 5 bars with no problem, made me a true believer in buffalo products, because their HP wireless router line has built in signal amp, makes the radios so much stronger than other brand routers. Not saying you can for sure do it with just 1 unit, but you'd be suprised.