As a Time Warner customer, what is the best modem to use with an Airport Extreme? Time Warner has "approved" various Motorola, netgear, Zoom and ZyXel modems.
It is important to go with a modem that the ISP approves. Personally, I have had no issues with those provided by Motorola.
Also can you link multiple Extremes or Express' together to extend the wireless range?
Yes, please check out the following Apple Support article for details.
I installed a Motorola Surfboard 6141 modem and an Airport extreme wifi router. The combo works great on Time Warner, and the wifi coverage is vastly improved in my house, which is a 100 year old ~4,000 square foot, three story residence.
With the modem and router on the first floor (in the front), I get strong signals on the 2nd and 3rd floors, as well as the back of the house.