I have a network in a school with about 200 PCs and 120 Macs. There is no problem with the PCs, sometimes things get a bit slow because all the students share an 24Mbit ADSL connection (actual bandwidth 19Mbps down, 2.1Mbps up) but there is no problem downloading files. However on the macs downloads frequently freezes, sometimes after 200-400 MB but sometimes after just a few MBs.
The macs where using a wireless connection. So I tried a PC on the wireless without a problem and then 3 different macs with cable and they got the same problem. I called apple support and got the tip to move a systemPreferences folder from the Library folder to the desktop, restart the computer so that OS X recreated it, however downloads still froze. I have tried safari, the new safari beta and Firefox but it's the same problem in all of them.
To get on the internet the computers have to go through a managed netgear switch then a computer running server 2003 and ISA 2004, a zywall 5 firewall and then the modem.