don't have to buy gigabit ethernet, but to get teh fastest internet speeds, then yea it's best you do. HOwever it woudl be recommened that you do so for all your computers and make sure you get cat5e ethernet cable not the regular cat5.
The reason I say upgrade all your computers to gigabit, is because being a consumer router, I would think that each port is not indipendant of one another, so the entire switch will fall back to the slowest link. So if you just got one gigabit NIC, you may only get 100Mb or less if the other hardwired computers on the lan only have 10/100.