Using a Router to Share Cable Connection

I am thinking of buying a router to share my Cable Modem connection with the four computers in my network. Before I do, however, I would like to know if there would be a loss of speed. Does the cable company limit bandwidth per ip or account? I am currently paying $15 extra a month for an extra IP. Also, I like to play wolfenstein over the internet at the sime time as my brother does on another computer on the network. Will this be possible with the single IP?