Yes, adding a external Gb network switch will help improve file xfer speeds between wired LAN devices. This helps to eliminate un-necessary ISP modem or router processing that can impact performance.
Recommended set up would look like this:
ISP modem>DIR-655>External Gb network switch<>All wired LAN devices connected to switch. The switch will provide the fastest performance between LAN wired devices and yet still each device can access the Internet.
I also connect higher bandwidth devices directly to the router that usually do not need any cross device access, like xbox gaming or maybe a PC that needs alot of bandwidth for downloading. I connect my 2 xboxes directly to the router so any major activity on the switch doesn't effect the gaming activities. Then I have a boxee box and a file server directly connected to the switch for seamless and smooth video streaming between these to devices.
My setup looks like this:
ISP modem>DIR-827><External Gb 24port network switch<>Wired LAN devices, boxee box, file server, and pcs connected to switch.
><2 xboxes connected directly to the DIR-827 router and not the switch.
I recommend an un-managed Gb network switch. Dlink has a few models to choose from and ports from 4 to 48 I think. Pick port usages on what needs are.
http://www.dlink.com/ca/en/business-solutions/switching/unmanaged-switches/desktopLet us know what you go with and end up getting.