TCP can be tuned quite a bit as the defaults don't always work well on things like high speed networks.
There are limits on how fast things can go. Have a look at this page 
http://wand.net.nz/perry/max_download.php
There are plenty of resources out on the web giving tuning advice:
http://www-didc.lbl.gov/TCP-tuning/linux.htmlhttp://www-128.ibm.com/developerworks/linux/library/l-hisock.html?ca=dgr-lnxw01BoostSocketSee also TcpWindow
2 pages link to TcpTuning:
