Here's the basic deal. if your computers are all gigabit capable, and are connected through a switch, router or hub which is also gigabit capable you will have 1000mbps connections from pc to pc.
BUT if the device through which you access the internet (router or modem) is only capable of 100mbps, then all data passing through it will travel at a max of 100mbps.
HOWEVER, todays 'normal' (heck, even awsome) internet speeds are not remotely as fast as standard networks, even at 100mbps. A 100mbps network will easily handle any traffic from your internet link without slowing it at all.