I purchased a new Arris Surfboard DOCSIS 3.0 Cable Modem. It is extremely fast.
Thought it was a Modem/Router. Have an old Netgear N300 Wireless Router can I use it or will it cause me grief. Or should I by a Netgear C3000 N300 8x4 Cable Modem/Router?

You can use the older router, but if you don't want to slow down the fast modem you want to upgrade to a new A router.

Thanks Scott, my next question is I can get a Modem/ Router for $89 dollars or a Modem only for $89 dollars. Speed is not necessary. Would you go for a cheaper Modem and hookup the old router? Also do you prefer two boxes over one box?

Personally I prefer separate boxes. First, Broadband modem technology has not advanced as fast as WiFi technology. The DOCSIS 3.0 spec was released in 2006. A 3.1 spec was released in 2013 with minor updates several times. These updates are rolled out to modem's by the cable provider or the you may have to get them from Arris.

On the other hand, the Wifi standards have gone through several major updates in that time.

Thanks again Scott. I went with 2 boxes and have it all set up and working fine.

Darn. I'm too late to the conversation. lol