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.