One other suggestion, you should test your network with something like jperf (iperf gui for windows). File transfers are not always a good judge of bandwidth limits because of all the dependencies involved. The problem could be with the sending/receiving hard drive, filesystem, OS, NIC, or some combination of those. If Jperf shows a good amount of bandwidth the bottleneck is not the network.
http://openmaniak.com/iperf.php#jperf - Jperf tutorial
Josh