Have 2wire 3800HGV homeportal. It has 4 port switch to connect computers to, but is only 10/100mbps. If I disconnect my computers from the switch and connect a router with the capacities of 10/100/1000mbps to the switch, and then connect my computers which have 10/100/1000mbps capacities to the router, will my computers communicate with each other at 1000mbps, or will it down grade to a slower speed?
Disable DHCP on the add-on router and plug all cables into the switch ports leaving the WAN port unused. You will essentially turn the router into a switch. Or better yet just get a 5 or 8 port Gig switch.
All Gig nic computers connected to the gig switch/router will communicate with each other at Gig speeds. Internet will still be limited to 10/100 or whatever speed your ISP has you set to.