I have for some time longed for a tool that would run in both Windows and Linux and be able to test the raw speed of the network between two endpoints. Now, half accidentally, I ran into this thread: Slow Gigabit Network - Win XP. And there it was: iperf.
With iperf I found that the bottlenecks were the actual pieces of software doing downloads, not the network itself. I managed to pull some 300+ megabits per second over TCP with gigabit link between nForce gigabit NIC and Netgear PCI NIC, and 700+ megabits between the said nForce NIC and Realtek TRL8111B integrated on Riitta’s Asus M2A-VM motherboard. Satisfactory to say the least.