If you get a 1Gbps switch, plug all your computers in it and only use the DG834 for the internet connection (through the switch's uplink port), then only your internet traffic will be limited to 100Mbps (which is usually more than enough). All the communication between the computers will be done at 1Gbps.
Exactly, that is the difference between a switch and a hub; a switch only sends data on a channel when the data need to be sent on that channel. The same thing happens with your current DG834 right now, nothing is sent through the WAN channel unless it needs to.