A D3 modem will bond multiple channels, IF bonding is available in your area, regardless of your tier. I believe performance is only 20/4 in areas with two way bonding, but it is possible to do it without.
Even if your HSI service tier doesn't require multiple bonded channels for its speeds, there are a number of advantages of having that: »Re: Need help buying dad a modem
Thanks to all for enlightening me. D3 modem is ordered! Will be burning in modem when it arrives and ordering service right after the first of the year.