Should I upgrade from a Motorola SB5120 (DOCSIS 2.0)?

For starters, your area has already had Preferred speeds upgraded to 50Mbps download speeds so you would see that immediate benefit if you upgrade the modem.  The DOCSIS 3.0 modems due to their channel bonding capabilities also provide for more consistent speeds during periods of peak demand.  You can find out more about modems approved for use on our network by visiting http://www.cox.com/residential/support/internet/article.cox?articleId=b2ec95d0-7ef9-11df-5590-000000000000