you can look up the specs for your cable modem online and see what its maximum speed is, and see whether or not you need a cable modem upgrade.
what I have seen so far (March 2016) in DOCSIS 3.0 modems is:
cable modem max speed calculator
why doesn't my internet go at the speed of my cable modem?
the tiered speeds offered by your ISP may be less than your modem's rated speed. buy what speed you can afford per month. it can get pricey.
the higher speeds may also cause unforseen problems with your equipment (such as pc lockups), or may work great. a high-end machine may eliminate the problem.
if you are working with any 1000Mbps stuff, it should alll be 1000Mbps, so examine specs closely when buying, and use cat5e cable (easier to deal with) or cat6 or cat6a.