The speed is controlled by the port you plug it into. As long as the cable is meets the specs for the port it will run at full speed. A cat5E rated cable can run up to 1gig. A cat7 cable which is rated to 10g will still only run at 1g if you connect it to 1g ports. Pretty much you can buy the cheapest cable you can find since almost everything is cat5e or cat6 you can find in the consumer market. You will see no difference in the performance between cat5E and cat6 because again it is the port in the machine that controls the speed not the cable. If you were to buy something like cat3 cable it would not run slower it would either get lucky and run at 1g speeds or completely fail.
If you want to get very technical cable length...