I recently had to test out the same scenario with my Boxee media server with my ReadyNas going from a 100mb intranet over to a gigabit intranet.
Like the thread that Dirge mentions, blue_bludger points out a tool called PCATTCP
which is very useful!
But I later opted with an app called LAN Speed Test v1.1.6 (Free)
as the other version v2.0.6
limits some functionality (but it's only $5.00!).