They are probably referring to the link rate. Older G only routers had a maximum link rate of 54Mbps. Newer routers that offer N have link rates of 100+ depending on the number of streams.
The biggest problem with older routers is they don't have the processing power and memory to handle today's higher speed connections. When I use my Linksys 54G behind my FIOS 75/35 connection it can't pass more than 30 - 40Mbps.
With a newer router your streaming will be be more reliable particularly if you plan on using WiFi.
As for the speed of the connection you need from your ISP I was able to stream Netflix in SD very well using just a 1.5Mbps connection. What is more important than absolute speed is consistent latency. If when your Roku calls for more data your ISP and/or the Internet needs to be able to deliver it right now. If your latency varies up and down this will result in buffering. The Roku has very little memory so it is dependent in just in time data delivery
Adding more speed to your internet plan and purchasing the most expensive router are just steps to eliminate some sources of potential delay.
Since Netflix will work very well with a 5 - 10 Mbps plan and a medium priced N type router such as FIOS provides you should have no problems streaming Netflix.
The biggest thing you can do to reliably stream Netflix or any other video is connect you Roku to your router using an Ethernet cable. If that isn't possible then use MOCA or Ethernet over powerline adapters.
You also might want to consider purchasing a newer Roku model that supports adaptive streaming.