Comcast has suggested I need to get a New modem. I get 30 mps download now according to speedtest. Wait, or buy a New DOCSIS 3 now?
When your cable company requires channel bonding, and unless I miss my guess that will be some time down the road.
As to whether to get a new modem now, 30 is about the max for a D2 device, if you subscribe to a faster internet package than 30 Mbps, then upgrading to a D3 modem will allow you to make user of that additional speed. So then the question becomes is the price worth the extra speed for what you do on the internet, and that is a finance question only YOU can answer.
Watching web pages load will be identical.
Watching streaming movies on one computer will be identical.
Watching streaming movies on multiple devices at the same time [kids and other household members] may get some benefit from the additional bandwidth.
Downloading LARGE files will get some benefit from the additional bandwidth.
Online games will not get an advantage unless you have people doing other things WHILE you are online gaming, then it may get some bandwidth protection from them.
Mostly its a bragging rights thing though most people do not currently use a 50Mbps of faster internet connection to its potential.
As to the "up to", call them up and ask them what your BASE speed is, that's sales talk and cable does not work like DSL where you lose bandwidth the further you get away from a point, it has distance limits where it will work, it will not work, or you can make it work with a house amp , that's the skinny of it. The real restriction on cable is the return and splitter config...
Speedtest's speed is what they call boost, and yeah its only for the first 10 Mb of a download, alter that you get throttled to your BASE speed. You can also use a tool like shaperprobe to try and figure out your base speed.
Packages speeds were recently upgraded for most areas, blast is now 50 Mbps, preferred is 25MBps IIRC, sorry I just repair the stuff, I never was good at sales.