Gigabit Ethernet . . .

is in regard to your home wired LAN. As in between your PC and another PC or device. If your router is Gigabit and the PC (and other devices) has a Gigabit Ethernet adapter, file transfer between devices will be improved. It will NOT affect your download speed from your ISP.

My Cisco cable modem and D-link router are Gigabit as are all my wired PCs. Transferring and moving files and such between my PCs is faster than the older 10/100 LAN speed. But my ISP's offering of 15 Mb/s is still 15 Mb/s.