Buying a new modem/router for gigabit internet and I do not know which one to buy.

I need a lot of upload speed so I am upgrading to gigabit internet. But with that, I need a new modem and router. The thing is, the descriptions of different gigabit router-modem combos say "best for up to 300mbs" or "best for up to 600mbs." But if they are gigabit modem/routers why are they only recommended for up to speeds lower than 1000mbs? Basically I am trying to find the least expensive router/modem that will still allow for the full potential use of my internet plan. Any help would be graciously appreciated.