For some time now we've been experiencing a problem across the board with Dell PowerEdge 2900/2950 servers equipped with Broadcom Gigabit ethernet cards. The symptoms usually manifest themselves in the form of slow upload speeds between certain endpoints. Dell Server to Dell Workstation transfers seem unaffected, however when using non Intel/Broadcome cards (Realtek, for example), upload speeds over a gigabit network come in around 100kbytes/second. We've also had this problem out to the internet when paired with both Cisco and Dell switches.
Dell Support has been pretty hush on this issue, failing to even admit it is a widespread problem (indeed if you Google about it you'll find lots of complaints).
TOE seems to be to blame and some posters have gone as far to recommend removing the TOE key (the hardware key that enabled the feature) and reinstalling the OS.
Indeed it does appear to be a problem with TOE and Broadcom drivers (among the other issues we have with them, but that's another story).
The good news is that there is a pretty simple fix for this, assuming you are running Windows 2003 SP2 with the Scalable Networking Pack (SNP):
Netsh int ip set chimney DISABLED
This disables TOE offloading, known as "TCP Chimney offload" in Windows.
We've run this on a number of servers now, and in all cases, it has resolved the slow transit issues.
Now only if Broadcom would fix their drivers.